Question

When 0.28g of a basic oxide, MO, is reacted with 250cm3 of 0.05moldm–3 hydrochloric acid the excess acid required 50cm3 of 0.05moldm–3 sodium hydroxide solution for neutralisation. Which one of the following is the relative atomic mass of M?

A 12
B 28
C 40
D 56

the answer is C but i have no idea how!

Answer 1

mol of excess acid: 50 cm ³x 0.05 M = 2.5 mlmol

mol of acid reacted: (250 cm³ x 0.05 M) - 2.5 mlmol = 10 mlmol = 10 x 10⁻³ mol

MM (molar mass) MO = mass : mol = 0.28 : 10 x 10⁻³ = 28

MM MO = Ar M + Ar O

28 = Ar M + 16

Ar M = 12

I think the answer is A