help on chem plzzzzz?

Question

hydrogen gas has been suggested as a clean fuel because it produces only water vapor when it burns. if the reaction has a 98.8% yield, what mass of hydrogen forms 85.0 kg of water?

Answer 1

THE ANSWER THE GUY GAVE ABOVE DOES NOT COMPENSATE FOR THE 98.8% PART. He'd be right if that wasn't there...So here's my take:

H2+ 1/2 O2--> H2O 85.0kg

Everything in chemistry works in molar ratios, so first step would be, how many moles of water was made?

Water is 2(1.008g)+16g=18.016g/mol
So 85x10^3g / 18.016g/mol= 4718 moles of water.

Now, that's how much you get at 98.8% actual yield, so to know how much stuff you need to make it, you need to find the theoretical yield, meaning the moles needed to make the desired end product amount in a perfect situation (100% efficient)

The equation is:

yield% = Moles produced (actual) / Moles calculated/desired (theoretical) * 100%

0.988= 4718/x
x= 4775 moles of water

Now in theory, to get 4775 moles of water, we need to follow the stoichiometry of the equation:

H2 + 1/2O2 --> H2O
So 1mol H2 per mol H2O
0.5 mol O2 per mol H2O

So that means we need 4775 moles of H2 and 2387.5 moles of O2. We measure things in grams though, so let's make these into required masses:

4775 moles H2 * 2(1.008)g/mol H2= 9626g H2 or 9.63 kg of H2
2387.5 mol O2* 32 g/mol O2 =76.4 kg of O2

So the mass of hydrogen needed is 9.63kg

Answer 2

Water is H2O. H has a weight that is 1/16 the amount of the mass of an O atom, so two Hs weigh 1/8th as much as O. So the total mass is (mass of hydrogen)/9mass of hydrogen + mass of oxygen) = 2/18 = 1/9th.

So 85.0kg divided by 9 = ~9.44 kg H2

Edit: --Good point, Calculust. Should account for yield. My quick n' dirty approach hath failed us all. :(

Answer 3

First write and balance the equation:

2H2 + O2 ---> 2H2O

Now convert the 85 kg of water to moles:
85/18 = 4.72 kmoles

2 kmoles of H2 give you 2 kmoles of H2O (from the reaction)
1 kmol H2 gives you 1 kmole H2O (simplification).
Because the reaction has yield 1 kmole of H2 will not be completely converted to water. The yield is 98.8% so

1 kmol H2 gives you 1*0.998 kmol H2

1 kmol H2 gives you 0.998 kmol H2O
Now for 4.72 mol of water we will have the following

x kmol H2 give you 4.72 kmol H2O

x = 4.72/0.998 = 4.73 kmol H2

Convert this to mass multiplying by the molecular weight of H2 = 2.016 kg/kmol

4.73 * 2.016 = 9.54 kg