## Knowing molecular formulas

NatalieSDis1A
Posts: 39
Joined: Fri Apr 06, 2018 11:05 am

### Knowing molecular formulas

For question E.9 in the homework it asks for the formula of magnesium sulfate heptahydrate. However, the question does not provide information (such as number of moles of each element used or percent composition) to find the molecular formula. Should I know from the chemical name what the molecular formula looks like? If so, is that something that has to be memorized?

Chem_Mod
Posts: 18400
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 435 times

### Re: Knowing molecular formulas

From the name, the formula can be obtained. The formula is $MgSO4\cdot 7H2O$. Sulfate is SO42-, so to get the ion charges to cancel with Mg2+, you need 1:1, thus MgSO4. This compound is hydrated, by 7 or "hepta", so you need 7H2O afterwards. This is explained in earlier parts of fundamentals section if you need to review it and is something that is expected to have been retained from high school chemistry.