# With complete combustion of 13.6 g of the substance, 28.4 g of phosphorus (V) oxide and 10.8 g

**With complete combustion of 13.6 g of the substance, 28.4 g of phosphorus (V) oxide and 10.8 g of water were obtained. 74 ml of a 32.0% by weight sodium hydroxide solution having a density p = 1.351 g / cm3 was added to the reaction products. What substance are you talking about in the task? What salt is formed as a result of this process and what is its proportion (in wt%) in the resulting solution?**

Given:

m (in-va) = 13.6 g m (P2O5) = 28.4 g m (H2O) = 10.8 g V (NaOH) = 74 ml ω (NaOH) = 32% ρ solution (NaOH) = 1.351 g / cm3 = 1.351 g / ml

Find: formula in-va -? salt formula -? ω (salt) -?

Solution:

1) Find the amount of substance P2O5: n (P2O5) = m (P2O5) / Mr (P2O5) = 28.4 / 142 = 0.2 mol;

2) Find the amount of substance P: n (P) = n (P2O5) * 2 = 0.2 * 2 = 0.4 mol;

3) Find the mass P: m (P) = n (P) * Mr (P) = 0.4 * 31 = 12.4 g;

4) Find the amount of substance H2O: n (H2O) = m (H2O) / Mr (H2O) = 10.8 / 18 = 0.6 mol;

5) Find the amount of substance H: n (H) = n (H2O) * 2 = 0.6 * 2 = 1.2 mol;

6) Find the mass H: m (H) = n (H) * Mr (H) = 1.2 * 1 = 1.2 g;

7) Find the mass O in in-ve (if any): m (O) = m (in-va) – m (P) – m (H) = 13.8 – 12.4 – 1.2 = 0 g ; This means that O was not contained in the matter;

8) Find the formula for the island P (x) H (y): x: y = n (P): n (H) = 0.4: 1.2 = 1: 3; This means that the unknown is PH3;

9) Write the reaction equation: P2O5 + 3H2O => 2H3PO4;

10) Find the amount of the substance H3PO4: n (H3PO4) = n (P2O5) * 2 = 0.2 * 2 = 0.4 mol;

11) Find the mass of NaOH solution: m solution (NaOH) = ρ solution (NaOH) * V solution (NaOH) = 1.351 * 74 = 100 g;

12) Find the mass of NaOH: m (NaOH) = ω (NaOH) * m solution (NaOH) / 100% = 32% * 100/100% = 32 g;

13) Find the amount of NaOH substance: n (NaOH) = m (NaOH) / Mr (NaOH) = 32/40 = 0.8 mol;

14) Write the reaction equation (taking into account the amount of the substance H3PO4 and NaOH): H3PO4 + 2NaOH => Na2HPO4 + 2H2O;

15) Find the amount of substance Na2HPO4: n (H3PO4) = n (Na2HPO4) = 0.4 mol;

16) Find the mass of Na2HPO4: m (Na2HPO4) = n (Na2HPO4) * Mr (Na2HPO4) = 0.4 * 142 = 56.8 g;

17) Find the mass of H3PO4: m (H3PO4) = n (H3PO4) * Mr (H3PO4) = 0.4 * 98 = 39.2 g;

18) Find the mass of the resulting solution: m solution = m (H3PO4) + m (NaOH) = 100 + 39.2 = 139.2 g;

19) Find the mass fraction of Na2HPO4 in solution: ω (Na2HPO4) = m (Na2HPO4) * 100% / m solution = 56.8 * 100% / 139.2 = 40.8%.

Answer: Unknown item – PH3; salt – Na2HPO4, the mass fraction of which was 40.8%.