Question

how much water should be added to 10 mL of 7% alcohol solution to reduce the concentration to 5%?

Answer 1

The volumes of the substances:
$V_{water}=10 \ mL \\ V_{alcohol}= 10 \ mL \times 7\%=10 \ mL \times 0.07=0.7 \ mL$

x - the amount of water that should be added

$\frac{0.7}{10+x}=5\% \\ \frac{0.7}{10+x}=0.05 \ \ \ \ \ \ \ \ \ \ \ |\times (10+x) \\ 0.7=0.05(10+x) \\ 0.7=0.5+0.05x \ \ \ \ |-0.5 \\ 0.2=0.05x \ \ \ \ \ \ \ \ \ \ \ |\times 20 \\ x=4$

4 mL of water should be added.