quantitative chem question gcse pls help

i would really appreciate help with this question, i keep getting 0.16 but the markscheme says 0.32 and i can't figure out why

Sulfur dioxide reacts with oxygen to produce sulfur trioxide:

2SO2(g) + O2(g) → 2SO3(g)

What mass of oxygen, in grams, would react completely with 1.28 g of sulfur dioxide?

Relative atomic masses (Ar): sulfur = 32, oxygen = 16

Reply 1

booklover1313

Original post by ourfavv

How many mols of sulfur dioxide do we have?

Reply 2

ourfavv

Original post by booklover1313

How many mols of sulfur dioxide do we have?

i think 2

Reply 3

ourfavv

that's what it says in the question

Reply 4

bl0bf1sh

can you think of a way to calculate the moles of SO2 using the data given?

Reply 5

booklover1313

Original post by ourfavv

that's what it says in the question

The 2 is the ratio in which it reacts.
We are told we have 1.28g.
The mass of one mol of sulfur dioxide would be 32 + 2(16) = 64
So mols = mass/Mr = 1.28/64 = 0.02

How many mols of Oxygen would react with 0.02 mols of sulfur dioxide, if they are in a 2:1 ratio of SO2 : O2?

Reply 6

ourfavv

Original post by booklover1313

isn't the mass 128 as there are 2 mols of sulfir dioxide so you would do 1.28/128 which is 0.01

(edited 1 year ago)

Reply 7

booklover1313

Original post by ourfavv

isn't the mass 128 as there are 2 mols of sulfir dioxide so you would do 1.28/128 which is 0.01

The 2 that you are talking about is only the ratio in which sulfur dioxide reacts with oxygen in this equation.
The mols we have is separate, and in 1.28g of sulfur dioxide, we would have the same number of mols, regardless of the ratios in any particular reaction equation.

The mass of 1 mol of sulfur dioxide is always 64g.
If we have 1.28g, that contains 0.02 mol of sulfur dioxide. That is always the case.

Reply 8

username5868172

so u have 1.28 of the sulfur dioxide, workout the mr of the sulfur dioxide which is 64 (32+16+16)
now use the equation: mass = mr x mole
do 1.28 divided by 64 which is 0.02
now you know the moles of sulfur dioxide
the ratio of moles of sulfur doxide to oxygen is 2:1
this means to workout the moles of oxygen you divide 0.02 by 2
which is 0.01
now you have the moles of oxygen, workout the mr of oxygen which is 32 (16+16)
now use the equation again to workout the mass :
32 x 0.01 = 0.32

Reply 9

username5868172

if u need more help pm me! i love quantitive chem

Reply 10

ourfavv

Original post by booklover1313

so this ratio does not effect the Mr?
it only tells you the ratio of the moles you have?

(edited 1 year ago)

Reply 11

booklover1313

Original post by ourfavv

so this ratio does not effect the Mr?

Exactly, the ratio is specific for each reaction equation and will vary depending on what the sulfur dioxide is reacting with.
The Mr is constant, because a mol is a certain number of molecules, which will always weigh the same.
So the only thing that the number of mols depends on is how much you have, so i.e. the mass.

Reply 12

Nazifa9936

Original post by ourfavv

Hello, A Level chem student here 😊

1. Find moles (n) of SO2
2. N (SO2) = mass/ Mr = 1.28/ (32+16+16) = 0.02mol
3. Find moles of O2 using ratio
4. The ratio between SO2 and O2 is 2:1, therefore if moles of SO2 are 0.02 then the moles of O2 must be half of that
5. N (O2) = 0.02/2 = 0.01mol
6. Find mass of O2 using mass = moles x Ar
7. Mass (O2) = 0.01 x (16x2) = 0.32 g
Note: I did 16 x 2 because Oxygen is diatomic. One oxygen = 16 but O2 is two oxygens, therefore its 16 x 2

Reply 13

bl0bf1sh

the "big numbers" in front of the formulae only represent the molar ratios, not the actual moles – 2SO2 + O2 tells you that O2 reacts with twice as many moles of SO2
this could mean that one molecule of O2 reacts with 2 molecules of SO2, 5000 mol of O2 reacts with 10000 mol of SO2, etc. ...

remember the Avogadro constant? in 1 mole of substance there are 6.022 x 10^23 particles

the equation for the reaction tells you the ratios in which the substances react, because
a) it would be silly/impractical to write the exact number of molecules or moles of each substance, and
b) the moles depend on how much (mass-wise) of each substance you use!

if you're making a cake, you could use 3 eggs, you could use 12 eggs, you could use 1 egg. you would adjust the quantities of the other ingredients accordingly so that the ratio of eggs : butter : sugar : flour is always the same

if that makes any sense?