# A level Chem mcq

Complete combustion of 0.050mol of an alkane produces 5.40g of H2O. What is the molecular formula of the alkane?
what to do after calculating mols of h2o as how are we supposed to know the molar ratio?
Original post by m4inotn4
Complete combustion of 0.050mol of an alkane produces 5.40g of H2O. What is the molecular formula of the alkane?
what to do after calculating mols of h2o as how are we supposed to know the molar ratio?

Calculate the moles of hydrogen atoms in 5.40 g of water.

You can then work out how many hydrogens are in each molecule by dividing said number of moles of hydrogen atoms by the number of moles of alkane used.

You should also know the general formula for an alkane is CnH(2n+2). Set the number of hydrogen atoms in each molecule equal to 2n+2 and solve for n.
oh wow yea! thank you I would never thought of that cool!
Original post by TypicalNerd
Calculate the moles of hydrogen atoms in 5.40 g of water.

You can then work out how many hydrogens are in each molecule by dividing said number of moles of hydrogen atoms by the number of moles of alkane used.

You should also know the general formula for an alkane is CnH(2n+2). Set the number of hydrogen atoms in each molecule equal to 2n+2 and solve for n.
Original post by TypicalNerd
Calculate the moles of hydrogen atoms in 5.40 g of water.

You can then work out how many hydrogens are in each molecule by dividing said number of moles of hydrogen atoms by the number of moles of alkane used.

You should also know the general formula for an alkane is CnH(2n+2). Set the number of hydrogen atoms in each molecule equal to 2n+2 and solve for n.

I'm still not getting the answer. Ive worked out moles of water came to be 0.3 then divided this by 0.05 and I got 6. and then 2n+2 = 6 wich gave n to be 2. the final answer in the marscheme is C5H12. I don't get it.
(edited 10 months ago)
Original post by minmin10
I'm still not getting the answer. Ive worked out moles of water came to be 0.3 then divided this by 0.05 and I got 6. and then 2n+2 = 6 wich gave n to be 2. the final answer in the marscheme is C5H12. I don't get it.

That’s because that’s the moles of water and not how many moles of hydrogen atoms in that amount of water.

Since each water molecule is H2O, there are 2H per molecule, so 2 x 0.3 mol = 0.6 mol
Original post by TypicalNerd
That’s because that’s the moles of water and not how many moles of hydrogen atoms in that amount of water.

Since each water molecule is H2O, there are 2H per molecule, so 2 x 0.3 mol = 0.6 mol

Ohh thta makes sense. and then u divide 0.6 by 0.05 ? Really sorry but if u don't mind could you do the full working out for it please?
Original post by minmin10
Ohh thta makes sense. and then u divide 0.6 by 0.05 ? Really sorry but if u don't mind could you do the full working out for it please?

That’s precisely it.

0.6/0.05 = 12
12 = 2n+2
10 = 2n
5 = n

So C5H12
Original post by TypicalNerd
That’s precisely it.

0.6/0.05 = 12
12 = 2n+2
10 = 2n
5 = n

So C5H12

Thankyou so much !
Original post by TypicalNerd
That’s precisely it.

0.6/0.05 = 12
12 = 2n+2
10 = 2n
5 = n

So C5H12

7 months late , but doing the question now and didn't understand it . Thanks for explaining