help on moles?

Question

I need help on these problems:

Calculate how many atoms of carbon are found in 7 formula units of iron(III) cyanide

For 125 grams of calcium sulfate dihydrate, how many moles are present

Answer 1

1) First, formula units are another way of saying molecules ("formula units" is a term usually used to indicate an ionic bond, where as "molecules" indicates a covalent bond). Second, recall that cyanide is a diatomic anion (CN-). Therefore, the molecular formula of the iron compound is: Fe(CN)3. Since there are 3 carbons per formula unit, 7 formula units would be 21 carbons.

2) The important thing here is to get the empirical formula correct. Using the periodic table, you can easily see that calcium (Ca) has a charge of +2. We know that the sulfate anion has a -2 charge, like in H2SO4. Therefore, these will 'cancel' out in the empirical formula: CaSO4 * 2 H2O . By using the periodic table, the mass of the compoud can be found and the moles can be easily found as well.

Answer 2

Not sure what a formula unit is!?

To calculate the number of moles of a substance, divide the mass by the molar mass of the substance (moles = mass / molar mass)

Calcium sulphate dihydrate (aka gypsum) is calcium sulphate with two molecules of water included, so it would be CaSO4.2 H2O. Now use your periodic table (or list of atomic weights) to add up the molar mass of calcium sulphate dihydrate
1 x Ca = ?
1 x S = ?
6 x O = ?
4 x H = ?
Molar mass CaSO4.2 H2O = ?

now to calculate the number of moles, divide the mass given by the molar mass you just calculated
moles = 125 / molar mass = ?

Answer 3

1.) 7 Formula units = 7Fe(CN)3
so...distribute like a math problem
7Fe
7C(3) = 21C
7N(3) = 21N

2.) 125g of CaSO4 2H2O (formula weight = 172.16816)

so.... (125/172.16816) = 0.726 moles