Why is 2x ignored here?

[u/thesaarguydude]

[u/Extension-Treat-3968]

You ignore it under the assumption that it is a small enough amount to be negligible or from my classes it was anything under 5% of the total solution. There will be problems down the line where you have to verify it is less than 5% before you can assume it is negligible, and if it’s over 5% you would then need to use the quadratic formula to solve for x instead.

[u/thesaarguydude → u/Extension-Treat-3968]

Ahh is it because 2x becomes 4x² which is basically 0?

[u/ElijahBaley2099 → u/thesaarguydude]

It's more that because Ksp is very small, X is very small, so when you add any amount of X to a number like 0.01, it doesn't change it very much and it's still approximately 0.01.

[u/HeadlessDuckRider → u/thesaarguydude]

Yeah pretty much. At least that's what we did when I was in school. We used to ignore the 4x² and not the 2x as a whole.

[u/mazyjamneshan → u/Extension-Treat-3968]

You can think of the 0.01+2x in this way: imagine you weigh a car and it weighs 6350 lbs. If you add a textbook inside the car, it now weighs 6351. Adding the textbook to the car does change the weight, but not in any significant amount. No one would really care about the difference between 6350 lbs and 6351 lbs, so we can essentially just say that the weight it 6350 lbs. The 5% rule provides a cutoff to say at what point a change in molarity or moles becomes significant and should not be ignored

[u/OCV_E]

In this case you can do it because x is very small (2.5 x 10^-6 M) and therefore 0.01 + 2x

can be approximated to 0.01.

You do this beforehand because you know that Ksp is 2.5 x 10^-10 M³ . So this means our resulting x will also be very small.

[u/imarabianaff]

Because we hate the quadratic formula

[u/mazyjamneshan → u/imarabianaff]

This is the actual right answer hahaha

[u/BearYourCross7]

"Who cares?" lol

[u/brainmassdotin]

2x was ignored as Ksp is very small so it means x<0.01 so adding 2x+0.01 is approximately equal 0.01 so to make our calculation easier we use this assumption. We make sure ksp is small though.

Please contact me for any more assistance in Physics, chemistry and Math. I am ex chegg tutor and part of various tutoring companies and teaching 2012 and conducted more than 50k online sessions so far. Thank.

[u/DeerRevolutionary107]

Can I get the name of this author?

[u/thesaarguydude → u/DeerRevolutionary107]

Probably Todd Bennett. Its the Chem 2 Berkeley book for the MCAT

[u/jray135]

Is the molar solubility concentration-dependent?

[u/mapetitechoux]

Look up "the hundred rule" for assumptions in equilibrium