expressing-the-concentration-of-a-solution MCQs for UPSC Prelims

26 practice questions on expressing-the-concentration-of-a-solution from the Exploring Mixtures and their Separation section of the UPSC Prelims syllabus. 26 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

7 Easy 13 Medium 6 Hard

Sample questions

Q1
easy
Two liquids that mix completely are combined, and the concentration is given as a volume by volume percentage. What does that percentage state?
  1. A The mass of one liquid present in $100\ \text{mL}$ of the mixture
  2. B The volume of one liquid present in $100\ \text{mL}$ of the mixture
  3. C The volume of one liquid present in $100\ \text{mL}$ of the other liquid
  4. D The volume of mixture obtained from $100\ \text{mL}$ of one liquid
Show answer and explanation

Correct answer: B - The volume of one liquid present in $100\ \text{mL}$ of the mixture

Volume by volume percentage divides the volume of one component by the volume of the whole mixture and multiplies by $100$. Using the other liquid as the denominator instead of the whole mixture always gives too large a figure. A mass in the numerator makes it a mass by volume percentage, and starting from one liquid and asking how much mixture results reverses the comparison.
Q2
medium
$30\ \text{mL}$ of a liquid flavouring is mixed with water to make $250\ \text{mL}$ of a drink. What is the volume by volume percentage of the flavouring?
  1. A $13.6\%\ \text{v/v}$
  2. B $30\%\ \text{v/v}$
  3. C $8.3\%\ \text{v/v}$
  4. D $12\%\ \text{v/v}$
Show answer and explanation

Correct answer: D - $12\%\ \text{v/v}$

The share is $\frac{30\ \text{mL}}{250\ \text{mL}} \times 100 = 12\%\ \text{v/v}$. Dividing by the $220\ \text{mL}$ of water instead of the whole drink gives about $13.6\%$. Dividing $250$ by $30$ reverses the ratio and gives about $8.3\%$, while $30\%$ simply repeats the volume of flavouring as though it were a percentage.
Q3
medium
A $200\ \text{mL}$ bottle of cough syrup is labelled $1.5\%\ \text{m/v}$ of the medicine. What mass of medicine does the full bottle hold?
  1. A $1.5\ \text{g}$
  2. B $3\ \text{g}$
  3. C $300\ \text{g}$
  4. D $0.75\ \text{g}$
Show answer and explanation

Correct answer: B - $3\ \text{g}$

The label means $1.5\ \text{g}$ of medicine in every $100\ \text{mL}$, and $200\ \text{mL}$ is twice that, so the bottle holds $3\ \text{g}$. Quoting $1.5\ \text{g}$ ignores the bottle size, and halving instead of doubling gives $0.75\ \text{g}$. The value $300\ \text{g}$ multiplies the volume by the percentage without dividing by $100$.
Q4
medium
A solution is made by dissolving $9\ \text{g}$ of a salt in $141\ \text{g}$ of water. What is the mass by mass percentage of the salt?
  1. A $9\%$
  2. B $94\%$
  3. C $6\%$
  4. D $6.4\%$
Show answer and explanation

Correct answer: C - $6\%$

The solution weighs $9\ \text{g} + 141\ \text{g} = 150\ \text{g}$, so the salt is $\frac{9}{150} \times 100 = 6\%$ of it. Using the $141\ \text{g}$ of water as the denominator gives about $6.4\%$. Quoting $9\%$ just repeats the mass of salt as a percentage, and $94\%$ is the share of the water.
Q5
easy
A bottle of antiseptic liquid carries the label $2\%\ \text{m/v}$. What does this tell a user?
  1. A Every $100\ \text{mL}$ of the liquid contains $2\ \text{mL}$ of the active substance
  2. B Every $100\ \text{mL}$ of the liquid contains $2\ \text{g}$ of the active substance
  3. C Every $2\ \text{mL}$ of the liquid contains $100\ \text{g}$ of the active substance
  4. D Every $100\ \text{g}$ of the liquid contains $2\ \text{g}$ of the active substance
Show answer and explanation

Correct answer: B - Every $100\ \text{mL}$ of the liquid contains $2\ \text{g}$ of the active substance

A mass by volume percentage gives grams of solute in $100\ \text{mL}$ of solution, so this label means $2\ \text{g}$ in $100\ \text{mL}$. Grams in $100\ \text{g}$ would be a mass by mass percentage, and millilitres in $100\ \text{mL}$ would be a volume by volume percentage. Packing $100\ \text{g}$ of solute into $2\ \text{mL}$ of liquid turns the label upside down.
Q6
medium
A $750\ \text{mL}$ bottle of floor cleaner is labelled $8\%\ \text{v/v}$ of the active liquid. What volume of the active liquid is in the full bottle?
  1. A $8\ \text{mL}$
  2. B $60\ \text{mL}$
  3. C $94\ \text{mL}$
  4. D $600\ \text{mL}$
Show answer and explanation

Correct answer: B - $60\ \text{mL}$

Every $100\ \text{mL}$ holds $8\ \text{mL}$ of the active liquid, and $750\ \text{mL}$ is $7.5$ such portions, so the bottle holds $8 \times 7.5 = 60\ \text{mL}$. Quoting $8\ \text{mL}$ ignores the bottle size, dividing $750$ by $8$ gives about $94\ \text{mL}$, and $600\ \text{mL}$ takes $80\%$ of the bottle instead of $8\%$.
Q7
medium
A nurse prepares $500\ \text{mL}$ of a $0.9\%\ \text{m/v}$ salt solution. What mass of salt does this volume contain?
  1. A $9\ \text{g}$
  2. B $45\ \text{g}$
  3. C $0.9\ \text{g}$
  4. D $4.5\ \text{g}$
Show answer and explanation

Correct answer: D - $4.5\ \text{g}$

A $0.9\%\ \text{m/v}$ solution holds $0.9\ \text{g}$ of salt in every $100\ \text{mL}$, and $500\ \text{mL}$ is five such portions, so it holds $0.9 \times 5 = 4.5\ \text{g}$. Quoting $0.9\ \text{g}$ ignores the volume entirely, $9\ \text{g}$ multiplies by ten rather than five, and $45\ \text{g}$ is ten times the correct value.
Q8
easy
A label states a concentration as a mass by volume percentage. What does that figure give?
  1. A The mass of solvent, in grams, present in $100\ \text{mL}$ of solution
  2. B The mass of solute, in grams, dissolved in $100\ \text{mL}$ of solution
  3. C The volume of solute, in millilitres, present in $100\ \text{mL}$ of solution
  4. D The mass of solute, in grams, dissolved in $100\ \text{g}$ of solvent
Show answer and explanation

Correct answer: B - The mass of solute, in grams, dissolved in $100\ \text{mL}$ of solution

Mass by volume percentage puts a mass of solute over a volume of solution, and the figure is read as grams in $100\ \text{mL}$ of solution. Grams of solute in grams of solvent is a different measure altogether, and millilitres of solute in $100\ \text{mL}$ of solution is volume by volume percentage. The mass of solvent is never what a concentration label reports.

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