Element E in group 7 of the periodic system. Mass fraction of oxygen in its oxide is 61.2%. Determine the element.

Given:
Oxide – E2Oy
ω (O in E2Oy) = 61.2%

To find:
E -?

Decision:
1) Ar (E) = x;
Mr (E2Oy) = Ar (E) * N (E) + Ar (O) * N (O) = x * 2 + 16 * y = 2x + 16y;
2) Mr (E2Oy) = N (O in E2Oy) * Ar (O) * 100% / ω (O in E2Oy) = y * 16 * 100% / 61.2% = 1600y / 61.2;
3) 2x + 16y = 1600y / 61.2;
122.4x + 979.2y = 1600y;
122.4x = 620.8y;
x = 5.07y;
4) y = 1 x = 5.07 – there is no element with such an atomic mass;
y = 2 x = 10.14 – there is no element with such an atomic mass;
y = 3 x = 15.21 – there is no element with such an atomic mass;
y = 4 x = 20.28 – there is no element with such an atomic mass;
y = 5 x = 25.35 – there is no element with such an atomic mass;
y = 6 x = 30.42 – there is no element with such an atomic mass;
y = 7 x = 35.49 – Cl – chlorine.

Answer: Unknown chemical element – Cl – chlorine.