Question

An experimental procedure requires 10 ml of acid to be dissolved into a beaker of water. An additional 5 ml of acid is added to the solution. Which statement best describes the new solution?

Answer 1

Long story short, C is the answer.

Answer 2

If we dissolve 10 mL of acid into a beaker of water. The acid will be diluted.
Since the number of moles of acid will not change, we can use this formula:
M1 V1 = M2 V2

Where M is the concentration of the acid in the solution
V1 = 10 mL
and
V2 = Vwater + 10

If we add an additional 5 mL of acid, the new concentration of the acid will be:
M = (M1 (5mL) + M2 (Vwater + 10mL)) / (5mL + Vwater + 10mL)

The concentration of acid in the new solution will increase.