The element of group 5 of the periodic element forms the acid H3EO4 with a relative molecular weight of 142.

Given:
E – element of group 5
Mr (H3EO4) = 142

To find:
E -?

Solution:
1) Using the periodic system of D.I.Mendeleev, calculate the relative atomic mass of the element:
Mr (H3EO4) = Ar (E) * N (E) + Ar (E) * N (E) + Ar (O) * N (O);
1 * 3 + Ar (E) * 1 + 16 * 4 = 142;
Ar (E) = 75 – arsenic (As).

Answer: The unknown element is arsenic (As).