Chemical element (X) forms with hydrogen H3X, the mass fraction of hydrogen is 8.827%.

Given:
ω (H in H3X) = 8.827%

To find:
X -?

Decision:
1) Calculate the relative molecular weight of H3X:
Mr (H3X) = N (H in H3X) * Ar (H) * 100% / ω (H in H3X) = 3 * 1 * 100% / 8.827% = 33.9866 ≈ 34;
2) Calculate the atomic mass X:
Ar (X) = Mr (H3X) – Ar (H) * N (H) = 34 – 3 * 1 = 31;
3) Using the periodic system of D.I.Mendeleev, determine the unknown chemical element X:
this is phosphorus – P;
4) Unknown compound – H3P – phosphine.

Answer: Unknown chemical element X – phosphorus – P.