Calculate the mass of P2O5 required to obtain 3.92 g of H3PO4.

Let’s write the reaction equation:

P2O5 + 3H2O = 2H3PO4

1.M (H3PO4) = 3 * 1 + 1 * 31 + 4 * 16 = 98 g / mol (found from the periodic table);

2.n (H3PO4) = m / M = 3.92 / 98 = 0.04 mol (n is the number of moles of phosphoric acid);

3.from n (P2O5) = 1 mol, it turns out n (H3PO4) = 2 mol (according to the equation, coefficients in front of the formulas), then, to obtain n (H3PO4) = 0.04 mol, n (P2O5) = 1 * 0.04 / 2 = 0.02 mol;

4.m (P2O5) = n (P2O5) * M (P2O5) = 0.02 * 142 = 2.84 g;

M (P2O5) = 2 * 31 + 5 * 16 = 142 g / mol. (Found from the periodic table)