Tuesday

February 9, 2016
Posted by **David** on Monday, March 5, 2012 at 10:52pm.

N2(g) + O2(g) <-> 2NO(g)

If more NO is added, bringing its concentration to 0.800 M, what will the final concentration of NO be after equilibrium is re-established?

Kc = 0.800 M/[0.100M][0.100M] = 80 M?

- Chemistry -
**David**, Monday, March 5, 2012 at 10:53pmthe extra NO being added threw me off..

or is it supposed to be like Kc = 0.800M^2/[0.100M][0.100M] = 64 M?

- Chemistry -
**DrBob222**, Monday, March 5, 2012 at 11:06pmNo and no.

First, since the values quoted are equilibrium values, we need to calculate Kc (by the way, 80 is not an equilibrium value).

............N2 + O2 ==> 2NO

equil.......0.1..0.1.....0.5

Kc = (NO)^2/(N2)(O2)

Kc = (0.5)^2/(0.1)(0.1)

Kc = 25 is what I have.

Then we set up another ICE chart.

............N2 + O2 ==> 2NO

initial.....0.1...0.1....0.8

change.....+x.....+x.....-2x

equil......0.1+x..0.1+x...0.8-x

Kc = 25 = (NO)^2/(N2)(O2)

25 = ((0.8-2x)^2/(0.1+x)(0.1x)

Solve for x.

By the way, you need NOT go through a quadratic if you notice that you can take the square root of both sides.

- Chemistry -
**David**, Monday, March 5, 2012 at 11:22pmwhy does the equil end up being 0.8-x instead of 0.8 - 2x? and i solved for x but it gave me 2 different answers did I do something wrong?

- Chemistry -
**DrBob222**, Tuesday, March 6, 2012 at 12:56amFirst, it wasn't -2x because I made a typo.It's obvious from the initial value and the change value that it SHOULD be 0.800-2x and, in fact, I wrote (0.800-2x)^2 when I substituted into the K expression.

You ALWAYS get two answers when you solve a quadratic. Most of the time it is obvious which is the right one and which is the wrong one. For example, I would expect, although I didn't go through the math, if you take the two x values you have, multiply by 2 and subtract from 0.800 that one will be a positive number and one a negative number. The one giving a negative number when subtracted from 0.800 will be the one to throw away since you can't have negative concns.

- Chemistry -
**Dewi ,untari**, Tuesday, March 6, 2012 at 1:32amCngkreng jkt .

- Chemistry -
**Kassandra**, Sunday, February 16, 2014 at 11:37pmI don't know how to do the last algebra part that DrBob22 did not explain.

I have not taken algebra since 2 years ago it would have been helpful to see...

I haven't found out the math part anywhere!

- Chemistry -
**Tomas**, Thursday, September 10, 2015 at 6:17pmHow do we construct an ICE chart for the concns of the dissociated ions to form a precipitate so we can calculate the final concns of ions in solution?