GATE Overflow for GATE ESE - Recent questions in Principles of Water Chemistry

GATE ES 2026 | Question: 20

Mon, 23 Feb 2026 13:15:10 +0000

Which of the following options is/are CORRECTLY listing the species included in Total Kjeldahl Nitrogen (TKN) estimation of an aqueous sample?

Ammoniacal - $\text{N}$
Nitrite - $\text{N}$
Nitrate - $\text{N}$
Organic - $\text{N}$

GATE ES 2026 | Question: 31

Mon, 23 Feb 2026 13:14:49 +0000

The analysis of a water sample is shown in the table. The hardness of the water sample is $\_\_\_\_$ $\mathrm{mg} / \mathrm{L}$ as $\mathrm{CaCO}_{3}$.

Atomic weight $(\mathrm{g} / \mathrm{mol}): \mathrm{Ca}=40 ; \mathrm{Mg}=24 ; \mathrm{Na}=23 ; \mathrm{Cl}=35.5 ; \mathrm{S}=32 ; \mathrm{O}=16 ; \mathrm{N}=14$
$$\begin{array}{|c|c|}
\hline
\textbf{Species} & \textbf{Concentration } (\mathrm{mg/L}) \\
\hline
\mathrm{Na}^{+} & 30 \\
\hline
\mathrm{Ca}^{2+} & 10 \\
\hline
\mathrm{Mg}^{2+} & 15 \\
\hline
\mathrm{Cl}^{-} & 30 \\
\hline
\mathrm{SO}_{4}^{2-} & 60 \\
\hline
\mathrm{NO}_{3}^{-} & 5 \\
\hline
\end{array}$$

$87.5$
$62.5$
$25.5$
$47.5$

GATE ES 2026 | Question: 41

Mon, 23 Feb 2026 13:14:33 +0000

The following two reactions describe the chemistry of $\text{S(IV)}$ in an aquatic system.

Under ideal conditions, the equilibrium constants for reactions $\text{(i)}$ and $\text{(ii)}$ are $1.3 \times 10^{-2} \mathrm{M}\left(\mathrm{K}_{\mathrm{S} 1}\right)$ and $6.6 \times 10^{-8} \mathrm{M}\left(\mathrm{K}_{\mathrm{S} 2}\right)$, respectively.

If the pH of the system is $4.0$ and the equilibrium concentration of $\mathrm{SO}_{2(\mathrm{aq})}$ is $1.0$ $\mathrm{M}$, the equilibrium concentration of $\mathrm{SO}_{3}^{2-}$ is $\_\_\_\_$ $\mathrm{mM}$ (rounded off to one decimal place).

$$\begin{array}{l}
\mathrm{SO}_{2(a q)} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HSO}_{3}^{-} \quad \text{(i)}\\
\mathrm{HSO}_{3}^{-} \rightleftharpoons \mathrm{H}^{+}+\mathrm{SO}_{3}^{2-} \quad \text{(ii)}
\end{array}$$

GATE ES 2026 | Question: 42

Mon, 23 Feb 2026 13:14:32 +0000

The calibration graph between absorbance and the concentration of chromate $\left(\mathrm{CrO}_{4}{ }^{2-}\right)$ in a water sample is shown in the figure.

For an absorbance of $0.35$ in a water sample, the estimated $\mathrm{Cr}(\mathrm{VI})$ concentration is $\_\_\_\_$ $\mathrm{mg} / \mathrm{L}$ (rounded off to two decimal places).

Use the atomic weight $(\mathrm{g/mol})$ of $\text{Cr}$ and $\text{O}$ as $52$ and $16$, respectively.

GATE ES 2025 | Question: 4

Thu, 06 Mar 2025 16:22:50 +0000

Choose the redox reaction from the following

$\mathrm{H}_{2} \mathrm{CO}_{3} \leftrightarrow \mathrm{H}^{+}+\mathrm{HCO}_{3}^{-}$
$\mathrm{Hg}^{2+}+2 \mathrm{OH}^{-} \leftrightarrow \mathrm{Hg}(\mathrm{OH})_{2}$
$\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2} \leftrightarrow 6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O}$
$\mathrm{CaCO}_{3}(\mathrm{~s}) \leftrightarrow \mathrm{Ca}^{2+}+\mathrm{CO}_{3}^{2-}$

GATE ES 2025 | Question: 26

Thu, 06 Mar 2025 16:21:49 +0000

A researcher added certain amount of $\mathrm{HgCl}_{2}$ to water at $\mathrm{pH} 10$. He calculated the expected concentration of mercury in the water. He asked his student to measure the concentration. His student used an instrument that can measure only the free metal, $\mathrm{Hg}^{2+}$. The student observed that the concentration measured by him was significantly less than the concentration calculated by the researcher. How can he explain the paradox to the researcher?

Explanation $1$: Significant fraction of the mercury would have changed phase from aqueous to gaseous phase, thus leaving less mercury in the water.

Explanation $2$: Fraction of the mercury added to water would have formed aqueous complexes.

Explanation $3$: Fraction of the mercury added to water could have precipitated.

Choose the correct option from the following:

only Explanation $1$ is correct
Explanations $1$ and $2$ are correct
Explanations $1$ and $3$ are correct
Explanations $2$ and $3$ are correct

GATE ES 2025 | Question: 27

Thu, 06 Mar 2025 16:21:46 +0000

Correctly label the speciation diagram below:

$\mathrm{I}=\mathrm{PO}_{4}{ }^{3-}, \mathrm{II}=\mathrm{HPO}_{4}{ }^{2-}, \mathrm{III}=\mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-}, \mathrm{IV}=\mathrm{H}_{3} \mathrm{PO}_{4}$
$\mathrm{I}=\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{II}=\mathrm{HPO}_{4}{ }^{2-}, \mathrm{III}=\mathrm{H}_{2} \mathrm{PO}_{4}^{-}, \mathrm{IV}=\mathrm{PO}_{4}{ }^{3-}$
$\mathrm{I}=\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{II}=\mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-}, \mathrm{III}=\mathrm{HPO}_{4}{ }^{2-}, \mathrm{IV}=\mathrm{PO}_{4}{ }^{3-}$
$\mathrm{I}=\mathrm{PO}_{4}{ }^{3-}, \mathrm{II}=\mathrm{H}_{2} \mathrm{PO}_{4}^{-}, \mathrm{III}=\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{IV}=\mathrm{HPO}_{4}{ }^{2-}$

GATE ES 2025 | Question: 33

Thu, 06 Mar 2025 16:21:27 +0000

Choose the correct option(s) from the following regarding the solubility in water

Water is a polar molecule because of the asymmetric distribution of charge between the oxygen and hydrogen atoms of the water molecule.
In a water molecule, the electrons shared between oxygen and hydrogen are attracted more towards the hydrogen atom.
Non-polar compounds are highly soluble in water because of their strong interaction with water molecules.
Aromaticity and charge of molecules influence their solubility in water.

GATE ES 2025 | Question: 55

Thu, 06 Mar 2025 16:20:52 +0000

Water from a hand pump located near a landfill has $1 \mathrm{mg} / \mathrm{L}$ arsenic (oral carcinogenic potency factor $=1.75(\mathrm{~kg-day}) / \mathrm{mg})$. A person who lives nearby drinks $2 \mathrm{~L} /$ day water from this hand pump for $10$ years. Assume body weight of $70$ kg and $70$ years as average life duration. Chances of this person getting excess risk of cancer is $\_\_\_\_\_\_$ $\times 10^{-3}$. (rounded off to three decimal places)

GATE ES 2024 | Question: 2

Mon, 19 Feb 2024 10:24:14 +0000

Hardness in water is NOT caused by

$\mathrm{Ca}^{2+}$
$\mathrm{Si}^{2+}$
$\mathrm{Mg}^{2+}$
$\mathrm{CO}_{3}^{2-}$

GATE ES 2024 | Question: 28

Mon, 19 Feb 2024 10:24:09 +0000

A saturated $\mathrm{CaCO}_{3}$ stock solution is existing at $25^{\circ} \mathrm{C}$. In one experiment $\text{(i)}$ $25 \mathrm{~g}$ $\mathrm{Na}_{2} \mathrm{CO}_{3}$ is added to the stock solution. In another experiment $\text{(ii)}$ $25 \mathrm{~g} \mathrm{Na}_{2} \mathrm{SO}_{4}$ is added to the stock solution. Select the correct statement from the following.

Addition of $\text{(i)}$ increases the concentration of $\mathrm{Ca}^{2+}$ and addition of $\text{(ii)}$ decreases the concentration of $\mathrm{Ca}^{2+}$
Addition of $\text{(i)}$ decreases the concentration of $\mathrm{Ca}^{2+}$ and addition of $\text{(ii)}$ increases the concentration of $\mathrm{Ca}^{2+}$
Addition of $\text{(i)}$ and $\text{(ii)}$ increase the concentration of $\mathrm{Ca}^{2+}$
Addition of $\text{(i)}$ and $\text{(ii)}$ decrease the concentration of $\mathrm{Ca}^{2+}$

GATE ES 2024 | Question: 40

Mon, 19 Feb 2024 10:24:07 +0000

The $\mathrm{pH}$ of a solution containing $0.1 \mathrm{M}$ of acetic acid and $0.05 \mathrm{M}$ of sodium acetate is __________ $\text{(rounded off to $2$ decimal places)}$.

The $\mathrm{pK}_{\mathrm{a}}$ value of ionization of acetic acid is $4.76$.

GATE ES 2024 | Question: 41

Mon, 19 Feb 2024 10:24:07 +0000

The ionic strength of a solution containing $0.01 \mathrm{M}$ of $\mathrm{CaCl}_{2}$ and $0.001 \mathrm{M}$ of $\mathrm{Na}_{2} \mathrm{SO}_{4}$ is _________ $\mathrm{M}$ $\text{(rounded off to $3$ decimal places)}$.

GATE ES 2024 | Question: 44

Mon, 19 Feb 2024 10:24:06 +0000

An industry discharges $2$ million liters per day $\text{(MLD)}$ of wastewater with a temperature of $45^{\circ} \mathrm{C}$ and a $\mathrm{pH}$ of $2$, whereas the neighboring industry produces $3 \mathrm{MLD}$ of wastewater with a temperature of $30^{\circ} \mathrm{C}$ and $\mathrm{pH}$ of $8$. If both the wastewaters are mixed and carried through a pipeline, then the resultant $\mathrm{pH}$ of mixed wastewater is _________ $\text{(rounded off to $2$ decimal places)}$.

Neglect buffering capacity of the system and the temperature effect on $\mathrm{pH}$.

GATE ES 2024 | Question: 53

Mon, 19 Feb 2024 10:24:05 +0000

A person consumes $2.5$ liters of water per day. The water quality test indicated that the supplied water has a $\mathrm{Pb}$ concentration of $0.6 \mathrm{mg} / \mathrm{L}$. If the weight of the person is $75 \mathrm{~kg}$, the exposure level for $\mathrm{Pb}$ for this person from this drinking water source is __________ $\mathrm{mg} / \mathrm{kg} /$ day $\text{(rounded off to $2$ decimal places)}$.

GATE ES 2023 | Question: 4

Tue, 23 May 2023 03:12:47 +0000

For testing alkalinity for a water sample, first phenolphthalein indicator is added. The water remains colorless. However, when a few drops of methyl orange is added to the sample, the color turns yellow. As per these observations, the correct choice is

Absence of $\mathrm{CO}_{3}{ }^{2-}$ and/or $\mathrm{HCO}_{3}{ }^{-}$but the presence of $\mathrm{OH}^{-}$ions in the sample
Presence of $\mathrm{CO}_{3}{ }^{2-}$ and/or $\mathrm{HCO}_{3}{ }^{-}$but the absence of $\mathrm{OH}^{-}$ions in the sample
Absence of $\mathrm{CO}_{3}{ }^{2-}, \mathrm{HCO}_{3}{ }^{-}$and $\mathrm{OH}^{-}$ions in the sample
Presence of $\mathrm{CO}_{3}{ }^{2-}, \mathrm{HCO}_{3}{ }^{-}$and $\mathrm{OH}^{-}$ions in the sample

GATE ES 2023 | Question: 40

Tue, 23 May 2023 03:11:59 +0000

The solubility of gas A is $16 \mathrm{mg} / \mathrm{L}$ in water and its vapor pressure is $0.042 \mathrm{~atm}$. at $25^{\circ} \mathrm{C}$. In a closed system, the gas phase concentration of $\mathrm{A}$ is $10^{-3} \mathrm{~mol} / \mathrm{L}$. Assuming Ideal gas constant $(\mathrm{R})$ value as $0.0821 \mathrm{~L}-\mathrm{atm} / \mathrm{mol}-\mathrm{K}$, the concentration of gas $\mathrm{A}$ in water at $25^{\circ} \mathrm{C}$ is ___________ (in $\mathrm{mg} / \mathrm{L}$, $\text{rounded off to two decimal places).}$

GATE ES 2021 | Question: 15

Tue, 14 Mar 2023 03:44:32 +0000

Chlorine is most effective as a water disinfectant at a $\text{pH}$ of _____________.

$6$
$8$
$10$
$12$

GATE ES 2021 | Question: 17

Tue, 14 Mar 2023 03:44:30 +0000

What is the $\text{pH}$ of a water sample having $\mathrm{H}^{+}$ concentration of $10$ $\mathrm{mg} / \mathrm{L}$?
The atomic weight of $\mathrm{H}$ is $1 \mathrm{~g} / \mathrm{mol}$.

GATE ES 2021 | Question: 31

Tue, 14 Mar 2023 03:44:18 +0000

A $200 \mathrm{~mL}$ sample of water has an initial $\mathrm{pH}=9$. In order to determine alkalinity, the sample was titrated using $0.02 \mathrm{~N} \mathrm{H}_2 \mathrm{SO}_4$ acid to an end point of $\mathrm{pH}=4.5$. In the titration, $25 \mathrm{~mL}$ of $0.02 \mathrm{~N}$ $\mathrm{H}_2 \mathrm{SO}_4$ acid was required. What is the total alkalinity of the sample in ' $\mathrm{mg} / \mathrm{L}$ as $\mathrm{NaHCO}_3$ '?
[Atomic weight (g/mol): $\mathrm{Ca}=40, \mathrm{Na}=23, \mathrm{H}=1, \mathrm{C}=12, \mathrm{~S}=32$, and $\mathrm{O}=16]$

$2$
$12$
$21$
$305$

GATE ES 2021 | Question: 34

Tue, 14 Mar 2023 03:44:08 +0000

The characteristics of a water sample are as follows: $\mathrm{Na}^{+}=92 \mathrm{mg} / \mathrm{L}$, $\mathrm{K}^{+}=19.5 \mathrm{mg} / \mathrm{L}, \mathrm{Ca}^{2+}=40 \mathrm{mg} / \mathrm{L}$, and $\mathrm{Mg}^{2+}=24 \mathrm{mg} / \mathrm{L}$. What is the sodium adsorption ratio (SAR) of the water sample which may be considered for irrigation purposes?
[Atomic weight $(\mathrm{g} / \mathrm{mol}): \mathrm{Na}=23, \mathrm{~K}=39, \mathrm{Ca}=40$, and $\mathrm{Mg}=24$ ]

$2.8$
$1.9$
$2.0$
$4.00$

GATE ES 2021 | Question: 39

Tue, 14 Mar 2023 03:44:00 +0000

In the context of water quality, which of the following is/are correct for the most probable number $\text{(MPN)}$ of a water sample?

The estimated organisms are gram negative.
It is based on the assumption of Poisson distribution.
It measures the exact number of microorganisms present in the sample.
It includes the quantification of pathogenic virus.

GATE ES 2021 | Question: 53

Tue, 14 Mar 2023 03:43:49 +0000

The molar concentrations $(M$, i.e. $m o l / L)$ of some ionic species in a water sample were estimated as follows:
$\mathrm{Na}^{+}=0.25{\mathrm{M} ; \mathrm{Ca}^{2+}}^{2}=0.12 \mathrm{M} ; \mathrm{Cl}^{-}=0.32 \mathrm{M} ; \mathrm{HCO}_{3}^{-}=0.05 \mathrm{M}$.
The ionic strength of this water sample is __________$\mathrm{M}$ $\text{(correct up to two decimal places)}$.

GATE ES 2021 | Question: 54

Tue, 14 Mar 2023 03:43:49 +0000

Excess amount of solid calcium sulphate $\left(\mathrm{CaSO}_4\right)$ was added to a pure water sample $(\mathrm{pH}=7)$ so that some solids remain undissolved at the equilibrium. The solubility product of $\mathrm{CaSO}_4$ is $2 \times 10^{-5}$ $\mathrm{mol}^2 / \mathrm{L}^2$. The molar concentration of $\mathrm{SO}_4^{2-}$ in this water sample at equilibrium will be ____________ $\mathrm{mol} / \mathrm{L}$ $\text{(rounded off to three decimal places)}$.

GATE ES 2022 | Question: 5

Fri, 17 Feb 2023 07:06:13 +0000

Specific conductance is used in water analysis to indirectly estimate dissolved solids. The measurements used in the method accounts for

Cations only
All ions
Anions only
Un-ionized species

GATE ES 2022 | Question: 11

Fri, 17 Feb 2023 07:06:09 +0000

Carbon dioxide is used in recarbonation process. A solution has $1 \mathrm{~mole} / \mathrm{L}$ supersaturated calcium carbonate. Estimated amount of carbon dioxide (in grams) needed to completely convert calcium carbonate to calcium ions in $1$ litre solution is

$44.000$ gram
$0.044$ gram
$40.000$ gram
$0.040$ gram

GATE ES 2022 | Question: 19

Fri, 17 Feb 2023 07:05:57 +0000

For effluents generated by a molasses based distillery and wood based pulp and paper industry, which of the following statement(s) is/are NOT correct?

Both the effluents have dark colour.
Both the effluents have high toxicity.
Both the effluents generally have BOD greater than $15,000$ ppm.
Both the effluents have high $\mathrm{pH}$.

GATE ES 2022 | Question: 45

Fri, 17 Feb 2023 07:05:40 +0000

The Henry's law constant of $\mathrm{CO}_{2}$ is $3.4 \times 10^{-2} \mathrm{M}$ per atm in neutral water at $25^{\circ} \mathrm{C}$. The dissolved $\mathrm{CO}_{2}$ (i.e., $\mathrm{CO}_{2} \cdot \mathrm{H}_{2} \mathrm{O}$ ) at equilibrium undergoes the following reactions.
\[
\begin{array}{l}
\mathrm{CO}_{2} \cdot \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HCO}_{3}^{-} \quad\left(\text { Equilibrium constant }=4.3 \times 10^{-7} \mathrm{M}\right. \text { ) } \\
\mathrm{HCO}_{3}^{-} \rightleftharpoons \mathrm{H}^{+}+\mathrm{CO}_{3}^{2-} \quad\left(\text { Equilibrium constant }=4.7 \times 10^{-11} \mathrm{M}\right)
\end{array}
\]
If ambient $\mathrm{CO}_{2}$ concentration is $300 \mathrm{ppm}$, the total dissolved $\mathrm{CO}_{2}$ (in $\mu \mathrm{M}$, rounded off to one decimal place) is ________

GATE ES 2022 | Question: 44

Fri, 17 Feb 2023 07:05:40 +0000

A solution has $0.001$ mole/L zinc ions having $\mathrm{pH}$ of $6$. Solubility product of zinc hydroxide is given as $8 \times 10^{-18}(\mathrm{~mole} / \mathrm{L})^{3}$. Ignoring activity corrections, the ratio (rounded off to two decimal places) of reaction coefficient to solubility product would be