# College Chemistry

posted by .

How many milliliters of 1.55 M NaOH must be added to 125 mL of .19 M NaH2PO4 to make a buffer solution with a pH of 6.90?

I have attempted to solve this with no success, as this is an online homework assignment that requires an exact answer.
Hoping that you will be able to help. Thanks.

• College Chemistry -

80.6 milliliters? O.o

• College Chemistry -

I'm sorry but this answer didn't work...

• College Chemistry -

......H2PO4^- + OH^- ==> HPO4^-2 + H2O
start..23.75 .....0.........0.........0
change..-x........x.........x.........x
end...23.75-x.....0........x...........x
Substitute this into the (B/A) = 0.5036 equation below.

[mmoles H2PO4 at start is from 125 mL x 0.19 M = 23.75 mmoles.
x = mmoles OH that must be added.]

What is the ratio of base to acid?
6.90 = pk2 + log[(base)/(acid)]
I looked up k2 for H3PO4 and found 6.34E-8 for pK2 = 7.2 but you need to use the pKa in your work, especially if your data base requires an exact answer. Also, you need to confirm all of my work and round to satisfy appropriate significant figures requirements. Use the Henderson-Hasselbalch equation.
pH = pKa + log[(base)/(acid)
6.90 = 7.2 + log(B/A).
(base)/(acid) = 0.5036 or
base = 0.5036*acid

base is x from line 3 above. acid = 23.75-x.
x = 0.5036(23.75-x)
Solve for x.
I get something like 7.95 mmoles for the amount of NaOH to add.

How much 1.55 M NaOH is needed to give 0.00795 moles. M = moles/L. Solve for L and I get 0.00513 L or 5.13 mL.

Then you need to check this to make sure it produces a pH of 6.90.
5.13 mL x 1.55 M NaOH = 7.95 mmoles.
This will form 7.95 mmoles HPO4^-2(base)
This will leave 23.75-7.95 = 15.80 mmoles H2PO4^- (acid); then,
pH = 7.2 + log (7.95/15.8) = 6.90
Check my thinking. Check my work. Confirm all of this.

## Similar Questions

1. ### college chem

a. According to the Henderson-Hasselbalch equation, how many grams of sodium acetate will you need to add to 125 mL of 0.100 M acetic acid to make a pH=4.74 buffer?
2. ### CHEMISTRY HELP!

a. According to the Henderson-Hasselbalch equation, how many grams of sodium acetate will you need to add to 125 mL of 0.100 M acetic acid to make a pH=4.74 buffer?
3. ### General Chemistry

Starting out with 50 mL of 0.20 M NaHCO3, calculate how many mL of 0.50 M NaOH solution to add to make 100 mL of approximately 0.10 M (total) buffer solution with a pH of 10.35. By adding NaOH, some of the NaHCO3 gets converted to …
4. ### General Chemistry

How many milliliters of 0.5M NaOH must be added to 200 mL of 0.1M NaH2PO4 to make a buffer solution with a pH of 6.9?
5. ### Chemistry

Calculate the change in pH if 0.050 g of solid NaOH is added to 250 mL of a buffer solution that contains 0.80 M NaH2PO4 and 0.17M Na2HPO4. I found the pH of the buffer solution to be 6.54.
6. ### Chemistry

How many milliliters of 1.0 M NaOH must be added to 200. mL of 0.10 M NaH2PO4 to make a buffer solution with a pH of 7.50?
7. ### Chemistry

How would you calculate the pH of the buffer if 1.0mL of 5.0M NaOH is added to 20.0mL of this buffer?
8. ### Chemistry

How would you calculate the pH of the buffer if 1.0mL of 5.0M NaOH is added to 20.0mL of this buffer?
9. ### Chemistry

A pH = 7.6 buffer is needed in the lab. This buffer is made by first dissolving 17.42 g K2HPO4 in 600 mL of water. What is the pH of this salt solution?
10. ### chemistry

How many milliliters of a 0.250 M acetic acid solution must be added to 50.0 mL of a 0.750 M NaOH solution to produce a buffer solution with pH = 4.500 ?

More Similar Questions