Titration question

[Golden-Future]

Advertisement - Members don't see this ad

10 mL 5 M calcium hydroxide is required to titrate 50 mL hydrochloric acid. Which of the following gives the initial concentration of the acid?

So I though that you find the moles of Ca(OH)2 and then that equals the moles of HCL. Then divide the moles of HCL by the volume of HCL.

(1 x 10^-2 ) (5 x 10^-1) = 5 x 10 ^-3 moles / 5 x 10 ^-2 L HCL = 0.1M

But the answer is apparently 1/5 M. I'm confused, does it have to do with the fact that its a strong acid being tit-rated and somehow half the moles of the base are needed.




EDIT: I get it its a 2:1 molar ratio of HCL to ca(OH)2

 

[SonhosDaVida]

I got 0.1mol/50 mL of HCl, which is 2 M concentration, not 1/5M. Can you explain how you got 1/5 M?

 

[rdyotz]

where is this question from?

 

[Golden-Future]

I got 0.1mol/50 mL of HCl, which is 2 M concentration, not 1/5M. Can you explain how you got 1/5 M?

where is this question from?

This is from Khan academy.
It is a 2:1 molar ratio of HCL to ca(OH)2.

Multiply moles of ca(OH)2 by 2 to get moles of HCL. (5 x 10^-1)

(1 x 10 ^-2) (10 x 10 ^-2) = 10× 10^-4 moles HCL

10 x 10^-4 or 1 x 10^-3 moles HCL

1 x 10^-3 / 5 x 10^ -3 = 1/5 M

 

[PlsLetMeIn21]

Did you mean 10 mL 0.5 M Ca(OH)2? That would make the answer 0.20 M rather than 2.0.

It doesn't seem like KA practice questions are helpful.

 

[rdyotz]

This is from Khan academy.
It is a 2:1 molar ratio of HCL to ca(OH)2.

Multiply moles of ca(OH)2 by 2 to get moles of HCL. (5 x 10^-1) Shouldn't this be 0.05mol Ca(OH)2 * 2 = 0.1 mol HCl?

(1 x 10 ^-2) (10 x 10 ^-2) = 10× 10^-4 moles HCL Why are you doing this step?

10 x 10^-4 or 1 x 10^-3 moles HCL

1 x 10^-3 / 5 x 10^ -3 = 1/5 M Shouldn't this be 5 * 10^-2 for 50mL

I'm not following you here, and want to make sure I know whats going on. This is what I do (being extra verbose to help follow along):

First convert everything to liters (I hate how scientific notation looks without superscripts, so sticking to decimals):
10mL = 0.01L
50mL = 0.05L

Then the math:
M = (mol/L) becomes mol = (M * L)
5M * 0.01L = 0.05mol Ca(OH)2
2mol HCl to 1 mol Ca(OH)2
so
0.05mol Ca(OH)2 * 2 = 0.1mol HCl
Back to M = mol/L:
0.1mol / 0.05L = 2M HCl

Not sure how they get 1/10 of what I am doing

I tried finding the problem on Khan and couldn't can I get the link?

Note: You could also simplify the calculations since you are just looking to find the exact molar ratio of H+ to OH-
IE: mol OH- = mol H+, so:
M * L (OH-) = M * L (H+)
5M * 10mL * 2 (Account for 2 OH)= x * 50mL = 2M HCl
Since mL will cancel, no need to convert to liters first.

 

[SonhosDaVida]

Why do you say KA questions are not helpful? You wouldn’t find chemistry questions like this on the MCAT?

 

[Golden-Future]

Yes, I did make a mistake in my reply. And I like roman numerals b/c it helps me work with the numbers easier.

Find the moles of Ca(OH)2: (1 x 10^-2 L ) (5 x 10^-1 M) = 5 x 10 ^-3 moles Ca(OH)2

That equals the moles of HCL but multiply by 2 b/c 2:1 molar ratio: 5 x 10 ^-3 moles x 2 = 1 x 10 ^-2 moles HCL

Now divide moles HCL by the liters of solution: 1 x 10 ^-2 moles HCL/ 5 x 10 ^-2 L HCL = 1/5M HCL

Here is the link: Titration questions
Its question #9

 

[Swagster]

I found KA passages helpful for P/S, but useless for the other sections. For the sciences, they were unrealistic and the explanations were often incomplete. Their videos can be helpful if you need a refresher on a topic, but you should stick to AAMC and reliable books for passages ad questions.