I do not understand this Chemistry work at all.....please help

I went in to get extra help and still dont get it, and my teacher knows this. It would be appreciated if someone could help with these questions?

5. Convert 9.14 mol of PCI5 to number of molecules
7.A compound has 40.68% carbon, 5.12% hydrogen, and 54.20% oxygen (by mass). Calculate its empirical formula.

I have more questions, but will try my best to answer these on my own.

9.14 mol times 6.022 x 10^23 molecules/mol = 5.50 x 10^24 molecules

There is a handy graphic here:

http://www.chemteam.info/Mole/Avogadro-N…

which summarizes the steps used in this type of problem. Make sure you know how to (1) go from grams to molecules and (2) figure out how many atoms of a given type are in the molecules. Like this:

In your problem 5, how many atoms of Cl are present in 9.14 mol of PCl5.

To answer that, you calculate molecules, then multiply the molecules by 5.

Empirical formula:

http://www.chemteam.info/Mole/EmpiricalF…

1) Assume 100 g of compound is on hand. This means:

Carbon: 40.68 g
Hydrogen: 5.12 g
Oxygen: 54.20 g

2) Calculate moles:

Carbon: 40.68 g / 12.011 g/mol = 3.3869
Hydrogen: 5.12 g / 1.008 g/mol = 5.079
Oxygen: 54.20 g / 16.00 g/mol = 3.3875

Do yo see how the C and O values are nearly the same?

3) Divide by smallest:

Carbon: 3.3869 / 3.3869 = 1
Hydrogen: 5.079 / 3.3869 = 1.5
Oxygen: 3.3875 / 3.3869 = 1

4) Find small, whole number ratio:

Carbon: 1 x 2 = 2
Hydrogen: 1.5 x 2 = 3
Oxygen: 1 x 2 = 2

Empirical formula is C2H3O2

5. Convert 9.14 mol of PCI5 to number of molecules
One mole of anything = 6.02 * 10^23 things = Avogadro’s number
9.14 moles * 6.02 * 10^23 molecules per mole =

7.A compound has, 5.12% hydrogen, and 54.20% oxygen (by mass). Calculate its empirical formula.

The subscripts in the empirical formula are the ratio of the moles of each element!
Number of moles = mass ÷ molar mass
Anytime that I have a percent problem, I let the total mass = 100 grams

40.68% carbon = 40.68 grams of C ÷ 12= 3.39
5.12% hydrogen = 5.12 g ÷ 1= 5.12
54.20% oxygen = 54.20 g ÷ 16 = 3.39

To get whole numbers divide each number of moles by the least number of moles
C = 3.39 ÷ 3.39 = 1
H = 5.12 ÷ 3.39 = 1.51
O = 3.39 ÷ 3.39 = 1

Now you multiply all 3 mole numbers by a whole number which will convert 1.5 to a whole number. So multiply by 2
C2H3O2

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