Question

Sodium chromate is used to protect iron from corrosion and rusting. determine its formula mass. 261.97 amu 238.98 amu 74.99 amu 161.97 amu 138.98 amu

Answer 1

This problem requires that you know nomenclature, to identify the formula of the compond from its name, and the way how to determine the formula mass once you know formula.

1) Nomenclature: Chromate

Chromate is an ion (anion) from the chromic acid.

Chromic acid is formed from Chromic oxyde with the atomic number 6.

Then Cr + O2 -> Cr2O6 = CrO3

CrO3 + H2O -> H2CrO4

H2CrO4 = 2H(+) + CrO4 (2-) <---- this is the ion chromate

Now you have to join the chromate ion with the sodium ion => Na2 (CrO4)

2) Now you can calculate the molar mass of the formula Na2(CrO4).

You must only muliply every subscript times the atomic mass of the element:

Na2 => 2 * 23 g/mol = 46 g/mol

Cr => 52 g/mol

O4 => 4 * 16g/mol = 64 g/mol

Formula mass = 46 g/mol + 52 g/mol + 64 g/mol = 162 g/mol

I used the numbers with less significant figures thant the problem required but the answer is evident. Also, g / mol = a.m.u = atomic mass unit.

Answer: 161.97 a.m.u