# Homework Help: Unanswered chemistry question (from a test)

1. Oct 3, 2005

### professor

the net ionic equation for the reaction of calcium bromide and sodium phosphate contains which of the following species:
a. Ca^2+(aq)
b. PO4^3-(aq)
c. 2Ca3(PO4)2(s)
d. 6NaBr(aq)
e. 3Ca2+(aq)

now for some reasoni went ahead and put 6NaBr which is now most obviously to me not the answer...(only partially because it had a red slash over it) could someone shw me which one it would be... and more usefully for the next test...why it is that one.

2. Oct 3, 2005

### TRCSF

I could be wrong...

CaBr2 + Na3PO4 -> NaBr + Ca3(PO4)2

balanced...

3 CaBr2 + 2 Na3PO4 --> 6 NaBr + 1 Ca3(PO4)2

3. Oct 3, 2005

### professor

i went and wrote them out, and think 3ca^2+(aq) is it... could someone confirm this, or disprove me?... that was soo much easier thean i origionally thought...darn, i think what it was attempting to ask was of the reactants which species was present...poorly worded question i guess, but yeah i do see where i got 6NaBr

Last edited: Oct 3, 2005
4. Oct 3, 2005

### professor

wait... i see now, thats the complete equation, the question was worded fine, the NaBr dident react, it would be taken out in the net ionic eq leaving 3Ca^2+ and PO4^3- i guess

Last edited: Oct 3, 2005
5. Oct 3, 2005

### GCT

would any precipitate be formed?

6. Oct 4, 2005

### Gokul43201

Staff Emeritus
Start with the balanced molecular equation : 3 CaBr2 + 2 Na3PO4 --> 6 NaBr + Ca3(PO4)2

Next write the complete ionic equation : 3Ca(2+) + 6Br(-) + 6Na(+) + 2PO4(3-) --> 6Na(+) + 6Br(-) + Ca3(PO4)2

The important thing to notice (asGCT pointed out) is that among all the compounds present, only the calcium phosphate is insoluble, and will not ionize.

From the above equation, cancel off common terms between the LHS and the RHS to write the Net Ionic Equation....and that will give you the answer you need.

7. Oct 4, 2005

### Gokul43201

Staff Emeritus
Close, but not completely right.