Question

The teacher instructed three students A, B and C to prepare $50\%$ (mass by volume) solution of sodium hydroxide $NaOH$ . A dissolved $50g$ of $NaOH$ in $100ml$ of water. B dissolved $50g$ of $NaOH$ in $100$ g of water while C dissolved $50g$ of $NaOH$ in water to make a solution of $100ml$ . Which one of them has made the desired solution?

Answer 1

Mass by volume percent can be defined as the amount of solute dissolved in the $100ml$ of the solution. Here keep in mind that we are talking about mass of solute dissolved in solution and not the solvent. Students generally get confused between the two.

Complete answer:
Keeping in mind the definition of mass by volume percent and looking at each option carefully.
We can say that student C prepared the correct mass by volume solution.
So, let’s look at the solution first.
As, $Mass{\text{ by volume \% = }}\dfrac{{Mass{\text{ of solute (g)}}}}{{Volume{\text{ of solution (ml)}}}} \times 100$
$Mass{\text{ by volume \% = }}\dfrac{{50}}{{100}} \times 100$
$= 50\% Mass{\text{ by volume}}$
Hence, student C took the right amount of sodium hydroxide and prepared the correct $= 50\% Mass{\text{ by volume}}$ .
Also, the student's solution was not correct because he already took $100ml$ of water and we need our solution in $100ml$ of solution. . Now if he will take this much amount of water beforehand only, then after the addition of solute in our solvent, the solution will obviously exceed than hundred millilitres. So, this mixture of solute and solvent will not five our desired concentration. So the student A is also the wrong answer for our question
Also the student B preparation will not yield $50\%$ mass by volume solution.as his solute and solvent concentration will not yield the desired concentration.
Therefore, the correct answer is student C.

Note:
Sodium hydroxide is a colourless, crystalline compound with a boiling point of ${1390^o}C$ . In common terms it is also known as caustic soda and is useful in the production of soaps and detergents and can also be used as bleaching powder with the chlorine. Also keep in mind that sodium hydroxide is highly corrosive in nature.