# A LEVEL CHEMISTRY - Amount of substances question help

A 20.0 cm3 sample of a 0.400 mol dm−3 aqueous solution of a metal bromide
(MBrn) reacts exactly with 160 cm3 of 0.100 mol dm−3 aqueous silver nitrate.
What is the formula of the metal bromide? (1 mark)

A. MBr
B. MBr2
C. MBr3
D. MBr4
Original post by Dan_Malik
A 20.0 cm3 sample of a 0.400 mol dm−3 aqueous solution of a metal bromide
(MBrn) reacts exactly with 160 cm3 of 0.100 mol dm−3 aqueous silver nitrate.
What is the formula of the metal bromide? (1 mark)

A. MBr
B. MBr2
C. MBr3
D. MBr4

Silver nitrate is AgNO3 and silver bromide is AgBr.

So therefore the ionic equation should be Ag^+ (aq) + Br^- (aq) --> AgBr (s)

What does this ionic equation tell you about how the moles of Ag^+ and Br^- are related?
Original post by TypicalNerd
Silver nitrate is AgNO3 and silver bromide is AgBr.

So therefore the ionic equation should be Ag^+ (aq) + Br^- (aq) --> AgBr (s)

What does this ionic equation tell you about how the moles of Ag^+ and Br^- are related?

Mmm 🤔 not sure
Original post by Dan_Malik
Mmm 🤔 not sure

Well, since the equation shows the silver and bromide ions react in a 1:1 ratio, that must mean that the moles of bromide ions in the solution must be the same as the number of moles of silver ions.

Try calculating how many moles of AgNO3 were used (this is the same as the number of moles of silver ions, since each AgNO3 contains one Ag^+ ion) and how many moles of MBrn were used.

From this, how might you work out how many bromide ions there are per MBrn?
(edited 2 months ago)
0.016mol of AgNO3, so same number of Ag^+ ions. 0.008mol of MBrn was used. Is it 0.016-0.008 = 0.008mol.
Original post by OHIfe
0.016mol of AgNO3, so same number of Ag^+ ions. 0.008mol of MBrn was used. Is it 0.016-0.008 = 0.008mol.

No.

If there are 0.016 mol of Ag^+, how many moles of Br^- ions must there be to react with them all?

And therefore, how many Br^- ions are there per MBrn?
(edited 2 months ago)
Is it 0.016 mol of Br^-? So 1 Br^- ion per MBrn.??
Original post by OHIfe
Is it 0.016 mol of Br^-? So 1 Br^- ion per MBrn.??

Original post by OHIfe
Is it 0.016 mol of Br^-? So 1 Br^- ion per MBrn.??

There are 0.016 mol of Br^-, but this does not mean there must be 0.016 mol of MBrn.

How many moles of MBrn are in the 20 cm^3 of 0.40 mol dm^-3 solution?
Original post by TypicalNerd
There are 0.016 mol of Br^-, but this does not mean there must be 0.016 mol of MBrn.

How many moles of MBrn are in the 20 cm^3 of 0.40 mol dm^-3 solution?

0.008mol.Oh, so 0.016/0.008 which is 2. So MBr2?
Original post by OHIfe
0.008mol.Oh, so 0.016/0.008 which is 2. So MBr2?

That is correct.
Original post by TypicalNerd
That is correct.

Really? Is there an easy way to spot how to do it or is it just that?
Original post by OHIfe
Really? Is there an easy way to spot how to do it or is it just that?

Yeah, it’s essentially the same process as you would use to find an empirical formula - only understanding why that is the case involves more thought and the method used to find the moles of each constituent element is different to normal.
Original post by TypicalNerd
Yeah, it’s essentially the same process as you would use to find an empirical formula - only understanding why that is the case involves more thought and the method used to find the moles of each constituent element is different to normal.

Thank You!!