Achiever Week 1 #10

[Tsii 2H]

When solving the quadratic equation, we get two answers for x, how do we know which x to plug in to solve for equilibrium molar concentration?

[Krutika_Joshi_3I]

One is usually negative and one is positive, use the positive value.

[AngelinaYuleiGao 1B]

This actually stumped me on the Achieve homework! I got two positive values and was convinced I did something wrong. I looked at my work for like... forty minutes, but couldn't find anything wrong with it. Instead of telling yourself one is supposed to be negative and one is supposed to be positive like the other response said, I would recommend asking yourself, which answer makes more sense?

I got two positive x values; 0.22 and 5.47. In this case, 5.47 wouldn't make sense; if my initial concentration of 2.91 decreased by 5.47, I would get a negative concentration, which can't happen. So, I went with 0.22, and it was correct!

If one x value is negative, then you can immediately assume it's not the one you're looking for. However, if both are positive, try each one out and decide which makes more sense!

Hope this helps :)

[Harsimer Bal 3K]

If both x values are positive, the correct value to use is usually the smaller x value because the larger x value will most likely be larger than the initial concentration of reactants, which is not possible.

[JasmineReyes-2K]

Hi,

I had this same question!

But I believe that we should always use our positive value over the negative value !

[705573518]

If you aren't sure which value to use, I'd suggest just plugging them in to decide what makes sense. If one of the numbers results in a negative molarity, you know that is incorrect and use the other value. If both values work, check your work because you might have made a mistake somewhere.

[Alexandra Gill]

When using the quadratic formula, you will get a positive and negative value. Always use the positive one.

[Tyler Triviz 2L]

Remember that negative equilibrium concentrations are not used, only the positive values.

[Jessica Li 1G]

Hi, I also got two positive values. However, if you plug both these x values into the equation to solve for concentration of the reactant and product, one of the x values will lead to a negative answer so that must be wrong. Therefore, the other smaller x value should be correct!

[Desiree Eshraghi 3A]

I know people say choose the positive x which is true. If BOTH were positive however, then you choose the x which can logically be subtracted/removed from the molecule which mols are being decreased by (a possible multiple of) x.

[Maxwell Yao]

In short, when you solve the quadratic equations for X after doing your ICE tables you DO NOT want to use the values that are negative OR greater than your initial concentration.

Hope this helps.

[Muryam_Hasan_2I]

Either use the value that's positive, or, if both values are positive, use the one that results in no negative values when you plug x into the equilibrium row of your ICE table.

[Misha_Khan_2H]

You should use the positive value if you get one positive and one negative value.
If both values are positive, use the smaller value.

[Nandini_Parmar_1I]

Usually use the positive value because that is representative of the change from equilibrium for each of the molecules involved the reaction.

[Omar Alami 3H]

It depends on what two values you get. There are normally two different instances. You will have one value is positive and one that is negative and of course the one that is negative will not be chosen as x as you can't have a negative concentration. The other instance is when you have two positive values, but one is too big and gets you a negative value when you plug it into an expression for the concentration of one of the terms.

[Crystal Ros 2L]

For these, I usually use my own judgement. If there is no negative value that we can immediately eliminate, there is usually an x value that will not make sense. For example, if an initial concentration is 1.00 and the two options for x-values are 2.0 and 0.2, then 2.0 can be eliminated. It's a similar concept to extraneous solutions. Hope this helps.

[Shay Crookall 2L]

One of them typically won't make sense given the context of the problem; typically it'll be >1

[EriMizuguchi2K]

Hi! Usually one of the values you get for X will be negative, so you would just use the positive value you get.

[danielle05]

Usually one value is negative and the other is positive, so you would just go for the positive value. On my homework problem though, I had two positive values: 5.90 and 0.24. I plugged in both values to the equilibrium portion of my ICE chart and found that 5.90 would make my NO2 value negative (my equation was 3.34-2x, so plugging in 5.90 for x would result in around -8.5), which doesn't make sense, so I went with 0.24. I think plugging the values into the equation to see which one results in a positive concentration helps.

[Jeffrey Vo 2A]

I just asked a similar question, but now another question I have is wondering whether on a test if we have two positive values that both very similar to each other as answer, which one is correct? However, I'm not sure if this mathematically possible.

[dahlia Faruque]

The x value that has a negative sign is not the answer because concentration can not have a negative value.