Units of Concentration

[loreben]

---

Members don't see this ad.

Hi,
Good morning SDNers 🙂. I was wondering if someone could please walk me through the mathematical analysis for solving this problem. Thanks 🙂

Question

What is the concentration of bromide ion in a solution in which 35ml of 0.4M NaBr is mixed with 50ml of 0.1M CaBr2?

 

[ilovemcat]

First find the number of moles of NaBr
1. Molarity x Volume = moles

Molarity = 0.4 M
Volume = 0.035 L
Therefore moles NaBr equals (M x V) = 0.014 moles

Now look at the balanced equation for this reaction:

Sodium Bromide (an ionic compound) completely dissociates:
NaBr ---> Na+ and Br-

There is a 1:1 ratio between NaBr and Br-
If you had 1 mole of NaBr, it would dissociate to produce 1 mole of Br-
If you had 2 moles of NaBr, it would dissociate to produce 2 moles of Br-
But instead you have only 0.014 moles. Therefore only 0.014 moles of Br- are produced.



Next find the number of moles of CaBr2
1. Molarity x Volume = moles

Molarity = 0.1 M
Volume = 0.010 L
Therefore moles CaBr2 equals (M x V) = 0.001 moles

Now look at the balanced equation for this reaction.

Calcium Bromide (an ionic compound) completely dissociates:
CaBr2 --> Ca2+ and 2Br -

There is a 1:2 ratio between CaBr2 and Br-
If you had 1 mole of CaBr2, it would dissociate to produce 2 moles of Br-
If you had 2 moles of CaBr2, it would dissociate to produce 4 moles of Br-
But instead you have only 0.001 moles. Therefore only 0.002 moles of Br- are produced (twice as much).



In total then, you have: 0.014 moles Br- (from NaBr) and 0.002 moles Br- (from CaBr2).
Together, this equals: 0.016 moles Br-

To find the combined molarity, you need to find the TOTAL volume.
0.035 L of NaBr was added to 0.010 L CaBr2, therefore the total volume should be: 0.045 L

Finally, the molarity is:
M = total moles Br- / combined volume
M = 0.016 / 0.045
M = 0.35 mol/L

 

[loreben]

Thanks a lot 🙂

 

[cartman1980]

First find the number of moles of NaBr
1. Molarity x Volume = moles

Molarity = 0.4 M
Volume = 0.035 L
Therefore moles NaBr equals (M x V) = 0.014 moles

Now look at the balanced equation for this reaction:

Sodium Bromide (an ionic compound) completely dissociates:
NaBr ---> Na+ and Br-

There is a 1:1 ratio between NaBr and Br-
If you had 1 mole of NaBr, it would dissociate to produce 1 mole of Br-
If you had 2 moles of NaBr, it would dissociate to produce 2 moles of Br-
But instead you have only 0.014 moles. Therefore only 0.014 moles of Br- are produced.



Next find the number of moles of CaBr2
1. Molarity x Volume = moles

Molarity = 0.1 M
Volume = 0.010 L
Therefore moles CaBr2 equals (M x V) = 0.001 moles

Now look at the balanced equation for this reaction.

Calcium Bromide (an ionic compound) completely dissociates:
CaBr2 --> Ca2+ and 2Br -

There is a 1:2 ratio between CaBr2 and Br-
If you had 1 mole of CaBr2, it would dissociate to produce 2 moles of Br-
If you had 2 moles of CaBr2, it would dissociate to produce 4 moles of Br-
But instead you have only 0.001 moles. Therefore only 0.002 moles of Br- are produced (twice as much).



In total then, you have: 0.014 moles Br- (from NaBr) and 0.002 moles Br- (from CaBr2).
Together, this equals: 0.016 moles Br-

To find the combined molarity, you need to find the TOTAL volume.
0.035 L of NaBr was added to 0.010 L CaBr2, therefore the total volume should be: 0.045 L

Finally, the molarity is:
M = total moles Br- / combined volume
M = 0.016 / 0.045
M = 0.35 mol/L

I was getting a slightly different answer, so wanted to confirm whether the bolded parameter is correct (or did i miss a step in deducing it)

 

[loreben]

The answer in the book is 0.28M. Was that what you got?

Thanks.

 

[ilovemcat]

The answer in the book is 0.28M. Was that what you got?

Thanks.

Woops, made a calculation error. Nice catch, Cart.
For CaBr2, the volume should instead be: 0.05L (not 0.01L)
and the moles of Br- is then: (0.1M x 0.05L x 2) = 0.01 moles

The total moles equals: 0.014 + 0.01 = 0.024
The total volume equals: 0.035L + 0.05L = 0.085L

The combined molarity is: 0.28 M

Amazing how 1 miscalculation could mess up the rest of the calculations 😛

 

[loreben]

Thanks 🙂