i need help with my chemistry homework..?

Question

1. Calculate the percentage composition, by mass, of sucrose, C12H22O11.

2. A hydrocarbon contains 75% carbon my mass. Calculate the emperical formula of the hydrocarbon.

3. 1 mole of hydrazine has a mass of 32g. Calculate the molecular formula of hydrazine.

any help from anyone please =] x

Answer 1

C = 12 x 12 = 144
H = 1 x 22 = 22
O = 16 x 11 = 176
Total = 342
C% = [144 / 342] x 100
H% = [22 / 342] x 100
O% = [176 / 342] x 100

75% C and 25% H
75 g C / 12 = 6.25 mol C 6.25 / 6.25 = 1
25 g H / 1 = 25 mol H 25 / 6.25 = 4
CH4


N2H4 = 32 g
empirical formula = NH2 [mass= 16]
Molecular formula = N2H4 [mass = 32]

Answer 2

!) to find percent composition you first find the amount of each element in the compound. You do this by multiplying the number of each type of atom by the atomic mass of that atom.

( don't have a calculator, these are estimates)
C 12x 12.011=144
H 22x 1.0079= 22
O 11x 15.999=176

Then you find the formula mass by adding up all of the total masses of the elements.
144+22+176=342

then you divide each of the total atomic masses by the formula mass and multiply by 100.

144/342 x100=42%C
22/342x 100=8% H
176/342 x 100= 50% O

These are estimates, I have no idea what the actual percent is, check with a calculator.

2) To find the empirical formula:
Hydrocarbons are made up of only H and C. Since there is 75% C, there is 25%H.

Chance the % to grams. There will be 75g c and 25gH. Then you divide these by their molar masses.

C 75/12.011=6
H 25/1.0079= 25

Take the smaller of those two numbers and divide by both.
6/6=1
25/6=4

so the formula would be CH4

Wish I could help you with the last one but I have no idea what the formula is or how to do it. Sorry.