GenChem Question ??

[Kilerfornia]

---

Members don't see this ad.

I am new on this website so i thought i'd give it a try. I am currently studying for the DAT and hopefully will be taking it Monday May 3rd.

I have this one general chemistry question that i always seem to get wrong so if anyone can help me out with a nice and thorough explanation of the following question, i'd be more than thankful


How many ml of 0.05M HCL are required to turn 20ml of 0.05M KOH into a solution of pH 2.00 ?

 

[UndergradGuy7]

I am new on this website so i thought i'd give it a try. I am currently studying for the DAT and hopefully will be taking it Monday May 3rd.

I have this one general chemistry question that i always seem to get wrong so if anyone can help me out with a nice and thorough explanation of the following question, i'd be more than thankful


How many ml of 0.05M HCL are required to turn 20ml of 0.05M KOH into a solution of pH 2.00 ?

Been asked here...
http://forums.studentdoctor.net/showthread.php?t=406065

Basically you know it takes 20 milliliters to neutralize it. Then you want to know how much it takes to get to pH of two.

So you do m1v1 = m2v2
and (initial H in M)(x volume of HCl) = (40+x total milliliters of solution)(10^-2 M of H+)

You get 0.4 and 20+0.4 = 20.4

 

[jh311]

isn't the answer 30ml?
m1v1 = m2v2
(.05)(x) = (.01)(40ml + x)
.05x = .4ml + .01x

solve for x:
.04x = .4

x= .4/.04 = 10ml

we already had 20ml of HCl to have a 1:1 neutral solution, so just add this to what you already have 20 + 10 = 30ml total

 

[UCB05]

isn't the answer 30ml?
m1v1 = m2v2
(.05)(x) = (.01)(40ml + x)
.05x = .4ml + .01x

solve for x:
.04x = .4

x= .4/.04 = 10ml

we already had 20ml of HCl to have a 1:1 neutral solution, so just add this to what you already have 20 + 10 = 30ml total

Good catch! The molar concentrations of acid and base are different between the two questions 😀 It'll take more to reach pH 2 than in the question we usually see on this board.

 

[Kilerfornia]

Thank you all for the replies im getting them right now 😀