Help with moles chemistry question

Hi, I was wondering if anyone would be able to help me with this question, because I get really confused with it.

17.1g of al2(SO4)3 aluminium sulfate was dissolved in water
Calulate the number of sulfate ions SO4 2- present in the solution.

Molar mass on al2(SO4)3 is 342g mol-1 and Avogadro Constant is 6X10 23 mol-1

Thanks

Reply 1

Stiff Little Fingers

21

Firstly, calculate the number of moles of Aluminium Sulphate you'd have.
Then, since it dissociates and the SO₄ ions are all we are interested in, calculate how many moles of those there would be in the solution (by the ratio of those to aluminium in the compound).
Finally, convert from moles to number of molecules by times the number of moles by avagadro's number.

Reply 2

UKBrah

3

Sorry but is this a CHEM1 question?

Reply 3

tree123

OP

8

yep its a moles calculation from unit 1

Reply 4

tree123

OP

8

I know this sounds stupid, but I get why you work out how many moles of Aluminium Sulphate you'd have which is 0.05.

How would you calculate how many moles of SO₄ ions there there would be in the solution (by the ratio of those to aluminium in the compound)??

Reply 5

manderton

13

Original post by tree123

I know this sounds stupid, but I get why you work out how many moles of Aluminium Sulphate you'd have which is 0.05.

How would you calculate how many moles of SO₄ ions there there would be in the solution (by the ratio of those to aluminium in the compound)??

al₂(SO4)₃

numbers in bold.

Reply 6

tree123

OP

8

so there 3 moles of sulphate ions?

Reply 7

Stiff Little Fingers

21

Original post by tree123

so there 3 moles of sulphate ions?

No. The ratio of Al to SO₄ is 2:3, so times the moles of aluminium sulphate by

\frac {3}{2}

to get the moles of SO₄^2-

Reply 8

tree123

OP

8

ok, that would give me 0.075 moles of sulphate ions
Multiply this by Avogardos constant is 4.515x10 to the power 23

and this isn't the correct answer, 9x10 to the power 22?

Original post by tree123

ok, that would give me 0.075 moles of sulphate ions
Multiply this by Avogardos constant is 4.515x10 to the power 23

and this isn't the correct answer, 9x10 to the power 22?

There are 3 moles of sulphate ions for every mole of aluminium sulphate.

Reply 10

tree123

OP

8

How do you know there are 3 moles of sulphate ions

Reply 11

Stiff Little Fingers

21

Original post by tree123

ok, that would give me 0.075 moles of sulphate ions
Multiply this by Avogardos constant is 4.515x10 to the power 23

and this isn't the correct answer, 9x10 to the power 22?

ah yes, sorry - charco is right. There's 3 SO₄ molecules in Al₂SO₄ so there's 3 moles of SO₄ per mole of Al₂SO₄ - just times the number of moles by 3, not by 3 halves.

Original post by tree123

How do you know there are 3 moles of sulphate ions

Because the formula unit is written to show two aluminium ions for every three sulphate ions:

Al₂(SO₄)₃

Hence, every 1 formula unit contains two aluminium ions and three sulphate ions.

Reply 13

tree123

OP

8

there's 0.05 moles of al2(so4)3

0.05 x 3 equals 0.15 moles of sulphate ions

How can there be more moles of sulphate ions (0.15) in the whole solution which there was only 0.05 moles?

Original post by tree123

there's 0.05 moles of al2(so4)3

0.05 x 3 equals 0.15 moles of sulphate ions

How can there be more moles of sulphate ions (0.15) in the whole solution which there was only 0.05 moles?

The solution is 0.05 molar in aluminium sulphate BUT it is 0.15 molar in sulphate ions.

A motorway has 1000 cars per kilometer, but as car has four wheels there are 4000 wheels per kilometer.

Reply 15

tree123

OP

8

Sorry. just trying to get my head around this...
Al₂(SO₄)₃
Shows two aluminium ions for every three sulphate ions
Hence, every 1 formula unit contains two aluminium ions and three sulphate ions.

How does this relate to number of sulphate moles

Original post by tree123

Sorry. just trying to get my head around this...
Al₂(SO₄)₃
Shows two aluminium ions for every three sulphate ions
Hence, every 1 formula unit contains two aluminium ions and three sulphate ions.

How does this relate to number of sulphate moles

3 = 3

Reply 17

tree123

OP

8

okay thanks for your help.