How much potassium hydroxide is required to completely neutralize 4.9 g of phosphoric acid?

1. We make a chemical equation, not forgetting to put down the coefficients:
3KOH + H3PO4 = K3PO4 + 3H2O.
2. Find the amount of substance H3PO4:
n = m / M = 4.9g / 98 g / mol = 0.05 mol.
3. Using the chemical equation, we find the amount of substance KOH:
n (KOH) / n (H3PO4) = 3: 1;
n (KOH) = 3 * 0.05 / 1 = 0.15 mol.
4. Find the mass of KOH:
m (KOH) = n * M = 0.15 mol * 56 g / mol = 8.4g.
Answer: m (KOH) = 8.4g.