Question

when preparing a concentrated nacl solution, a student dissolves 20 grams of nacl in 500 ml of water. after some time, they realize that they need a more concentrated solution.
which of these explains how the student can increase the concentration?
a. ○ remove nacl from the solution by evaporation.
b. ○ add more water to the solution without adding more nacl.
c. ○ add more nacl to the solution without changing the amount of water.
d. ○ add more water and nacl in equal proportions.

Explanation:

To increase the concentration of a solution (concentration = $\frac{\text{solute amount}}{\text{solution volume}}$ or $\frac{\text{solute amount}}{\text{solvent amount}}$ for aqueous solutions), we can either increase the solute amount or decrease the solvent amount (while keeping other factors constant).

• Option A: Removing NaCl (solute) would decrease the concentration, so A is incorrect.
• Option B: Adding more water (solvent) without adding solute would decrease the concentration (dilution), so B is incorrect.
• Option C: Adding more NaCl (solute) while keeping the amount of water (solvent) constant increases the ratio of solute to solvent, thus increasing the concentration. This is correct.
• Option D: Adding water and NaCl in equal proportions would not necessarily increase the concentration (it depends on the original concentration, but generally, it won't increase it as effectively as just adding solute or reducing solvent; in most cases, it would either keep concentration same or increase minimally, but definitely not as good as option C).

Answer:

C. Add more NaCl to the solution without changing the amount of water.