# Gchem: Acid and Bases

Discussion in 'DAT Discussions' started by Envision, Aug 8, 2006.

1. ### Envision Envisioning... 7+ Year Member

Joined:
Jun 14, 2005
Messages:
1,000
1
Status:
Dentist
Hi guys, ok so im having some trouble understanding titration problems. here are 2 examples.

1) What volume of HCl was added if 20 ml of 1M NaOH is titrated with 1 M HCl to produce a pH=2?

a 10.2 ml
b 20.2 ml
c 30.4 ml
d 35.5 ml
e none
i noticed that the molarity of both are equal, so they should probably neutralize each other if given the same volume. however, im not sure where to go from there.

2) A 10 ml sample of HCl solution has a pH of 2. what volume of water must be added in order to change pH=4?

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
2. ### dat_student Junior Member 5+ Year Member

Joined:
Apr 5, 2005
Messages:
2,068
0
Do a search on #1. I've probably solved it 10 times already

#2)
10^-2 * 10
______________ = 10^-4 (pH = 4 so [H+] = 10^-4)
10 + V

10^-1
_______ = 10 + V
10^-4

1000 = 10 + V

V = 990 ml

If you want to be more accurate you should consider that water itself has some H+. Since that amnt is small you can ignore it.

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
3. ### DonExodus Dentist in Virgin Islands 10+ Year Member

Joined:
Jul 6, 2006
Messages:
121
1
Status:
Dentist
I had always set it up like (.01)(.01)=(.0001)(V2) and solved from there, getting 100.

Ive seen you answer several questions in this format, but Im unclear how to follow it. I was just doing CV=CV, and I think this is a similar set up, but Im still lost.
Could you show me how you set those up please?
Thanks,
Don

4. ### dat_student Junior Member 5+ Year Member

Joined:
Apr 5, 2005
Messages:
2,068
0
# of moles of H+
______________ = molarity
total volume

# of moles of H+
_____________________________ = [H+] (i.e. concentration of H+)
volume of HCl + volume of water

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
5. OP

### Envision Envisioning... 7+ Year Member

Joined:
Jun 14, 2005
Messages:
1,000
1
Status:
Dentist

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
6. ### joonkimdds Senior Member 7+ Year Member

Joined:
Jun 30, 2005
Messages:
2,780
2
Status:
Pre-Dental
let me solve it with my own way, i hope this makes it easier

pH = concentration of H
pH=2 = 10^(-2) = 0.1 = very very acidic
pH=4 = 10^(-4) = 0.001 = less acidic which means u added water here.

M1V1=M2V2
(0.1)(10mL) = (0.001)(10ml+x)
how?
becuz. 0.1 = molarity of H before u add water and so it's the 10ml HCL with pH=2.
and then it becomes pH=4 = 0.001 when u add water, x amount to 10mL.

and X is how much u added water and if u solve for it, u get 0.99mL and
that's 990L.

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
7. ### gogiantsss 2+ Year Member

Joined:
Feb 4, 2007
Messages:
142
1
Status:
Pre-Dental
Remember that in the lab, never add water to acid!

8. ### joonkimdds Senior Member 7+ Year Member

Joined:
Jun 30, 2005
Messages:
2,780
2
Status:
Pre-Dental
why not?

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
9. ### Chga Member 7+ Year Member

Joined:
Jan 11, 2006
Messages:
346
86
Status:
Pre-Health (Field Undecided)
b/c its always acid to water

10. ### Lonely Sol cowgoesmoo fan! 10+ Year Member

Joined:
Dec 13, 2005
Messages:
684
1
Status:
Pre-Dental
A large amount of heat is released when strong acids are mixed with water. Adding more acid releases more heat. If you add water to acid, you form an extremely concentrated solution of acid initially. So much heat is released that the solution may boil very violently, splashing concentrated acid out of the container! If you add acid to water, the solution that forms is very dilute and the small amount of heat released is not enough to vaporize and spatter it.

So Always Add Acid to water, and never the reverse, unless you want to get ur instructors attention!

11. ### joonkimdds Senior Member 7+ Year Member

Joined:
Jun 30, 2005
Messages:
2,780
2
Status:
Pre-Dental

I am so glad I made my table members do everything for me

Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...

We’ve been on the Internet for over 20 years doing just one thing: providing career information for free or at cost. We do this because we believe that the health education process is too expensive and too competitive.

We believe that all students deserve the same access to high quality information. We believe that providing high quality career advice and information ensures that everyone, regardless of income or privilege, has a chance to achieve their dream of being a doctor.