You are Here: Home

# A2 Chemistry: Calculating concentration given Kc question help watch

1. I'm having a terrible time with this question. The answer is 0.897 mol dm–3 but I'm stuck at getting 0.93 for some reason. Our teacher told us that she is giving the same type of question in our test tomorrow so I am absolutely terrified. Here it is:

Phosphorus pentachloride decomposes as shown:
PCl5 (g) -----> PCl3 (g) + Cl2 (g)
Given that the equilibrium constant for this reaction is 11.5 mol dm-3 at 300 K, what is the equilibrium concentration of PCl3 when the concentration of PCl5 at equilibrium is 0.07 mol dm-3

Ideas? Any help would be appreciated
2. 11.5=[Cl2] * [PCl3]/0.07

let [PCl3] = x

11.5* 0.07 = x^2

x = sqrt (11.5* 0.07)

x = 0.897 mol/dm3
3. (Original post by Maker)
11.5=[Cl2] * [PCl3]/0.07

let [PCl3] = x

11.5* 0.07 = x^2

x = sqrt (11.5* 0.07)

x = 0.897 mol/dm3
why did you square the x? and what happened to the concentration of Cl2? why did it disappear from the expression?
4. (Original post by Tara1208)
why did you square the x? and what happened to the concentration of Cl2? why did it disappear from the expression?
Because the products are in equal concentrations

x*x = X^2
5. (Original post by Lord Nutter)
Because the products are in equal concentrations

x*x = X^2
How did you know that?
6. (Original post by Tara1208)
How did you know that?
Look at the equation for the decomposition ...
7. (Original post by charco)
Look at the equation for the decomposition ...
i still don't get it.. 😣 i get that the number of moles of PCL3 and Cl2 is same..but still how come their concentration would be same at equilibrium? 😔
8. (Original post by Tara1208)
i still don't get it.. 😣 i get that the number of moles of PCL3 and Cl2 is same..but still how come their concentration would be same at equilibrium? 😔
PCl5 --> PCl3 + Cl2
1mol --------------> x mol + x mol

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 7, 2018
Today on TSR

### Top unis in Clearing

Tons of places at all these high-ranking unis

### Why do women like men in suits?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE