Answer:
- If the unknown compound contains 34.8% Mn then 100g of this compound would contain 34.8g
- The molar mass of Mn is 54.938g/mol. (1 mole of Mn has a mass of 54.938g)
- 34.8g is therefore (moles = mass / formula mass) = 34.8g / 54.938g/molwhich is 0.633moles
We know that 100g of our compound would contain 0.633moles but we've been told there's at least 1 mole of Mn within 1 mole of the compound's true formula, so 100g is too small.
This means we would need have more than 100g to have a full mole of Mn: 1 / 0.633 times more.
This value multiplied by 100g gives 157.98. So with only 1 Mn the mass of 1 mole of this unknown compound is 157.98g/mol which is all the question wants to know, it doesn't ask us to work out what the actual formula is (thankfully) and only mentions the formula before it's put in water, not what forms in the water (like a complex).
