30 g of water and 15 g of NaOH were mixed. Find the mass fraction of the base in the solution.

The mass fraction of a substance in a solution shows how many grams of a solute is in 100 g of a solution.
Mass fraction of a substance is calculated by the formula:
W = m substance / solution * 100%
m solution = m substance + m water
In our case, the solute is sodium hydroxide (NaOH), a soluble base.
Given:
m (H2O) = 30g
m (NaOH) = 15g
W (NaOH) = x% Solution.
W (NaOH) = m (NaOH) / m solution
m solution = m (NaOH) + m (H2O) = 30g + 15g = 45g
W (NaOH) = 15/45 * 100% = 33%
Answer: W (NaOH) = 33%