# Chemistry

Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of the acid has three acidic protons. Estimate the pH, and the concentrations of all species in a 0.300 M phosphoric acid solution.
pKa1: 2.16
pKa2: 7.21
pKa3: 12.32

Hint: Convert the pKa values to Ka values.

1. 👍 1
2. 👎 0
3. 👁 4,096
1. .......H3PO4 ==> H^++ H2PO4^-
I........0.3......0......0
C........-x.......x.......x
E......0.3-x......x.......x

k1 = (H^)(H2PO4^-)/(H3PO4)
Substitute the E line into Ks expression and solve for x = (H^+), convert to pH.

........H2PO4^- ==> H^+ + HPO4^2-
From k1 ionization you see(H+) = (H2PO4^-)
k2 = (H^+)(HPO4^2-)/(H2PO4-)
and since (H^+) is in the numerator and H2PO4^- is is the denominator, they cancel and k2 = (HPO4^2-)

.......HPO4^2- ==> H^+ + PO4^3-

k3 = (H^+)(PO4^3-)/(HPO4^2-)
I would substitute (H^+) from the first calculation you did, (HPO4^-) from the second calculation, then solve for (PO4^3-).

1. 👍 0
2. 👎 2
2. How do you find the concentration of[OH-]? :C

1. 👍 0
2. 👎 0
3. Would it be
pOH = 14 - pH
Then, [OH-] = 10^-pOH
After finding [OH-] find the mol ratio?

1. 👍 1
2. 👎 0
4. To find [OH]
pOH =14 - pH
[OH-] = 10^ - pOH

mol ratio is not required

1. 👍 1
2. 👎 0
5. KA1= 6.9e-3
KA2= 6.2e-8
KA3= 4.8e-13
KA2 and KA3 are so small, only use KA1 for [H+]

H3PO4 -> H+ H2PO4
I .350 0 0
C -x +x +x
E .35-x x x
KA1= x^2/.35-x 6.9e-3
x= .046 = [H+]
pH= -log(.046)=1.34
[OH-]=1e-14/.046
=2.2e-13

[H3PO4]=(.35-.046)M
=.304M

H2PO4- ->H+HPO42-
I .046 .046 0
C -y +y +y
E .046-y .046-y y

KA2= y(.046+y)/(.046-y)
=6.2e-8
y=6.2e-8

[HPO4^2-]=6.2e-8M
[H2PO4^-]=.046-6.2e-8=.046M

HPO4^2- >H+PO4^3-
I 6.2e-8 .046 0
C -z +z +z
E 6.2e-8-z .046+z z

KA3= z(.0460/
(6.2e-8)-z=4.8e-13

z=6.5e-19M=
[PO4^3-]

1. 👍 4
2. 👎 0
6. substitute your M for the M found here (0.350)

1. 👍 0
2. 👎 0

## Similar Questions

1. ### chem

Arsenic acid (H3AsO4) is a triprotic acid with Ka1 = 5 10-3, Ka2 = 8 10-8, and Ka3 = 6 10-10. Calculate [H+], [OH -], [H3AsO4], [H2AsO4-], [HAsO42-], and [AsO43-] in a 0.20 M arsenic acid solution. i cant do this. i don't even

2. ### chemistry

Phosphorus pentachloride, PCl5, reacts with water to form phosphoric acid, H3PO4, and hydrochloric acid, HCl. PCl5(s) + 4H2O() → H3PO4(aq) + 5HCl(aq) (a) What mass of PCl5 is needed to react with an excess quantity of H2O to

3. ### Chemistry

Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of the acid has three acidic protons. Estimate the pH, and the concentrations of all species in a 0.300 M phosphoric acid solution. kPa1: 2.16 kPa2: 7.21

4. ### Chemistry

Using chemical equations, show how the triprotic acid H3PO4 ionizes in water. Phases, such as (l) or (aq), are optional.

1. ### AP CHEM

You need to use KH2PO4 and K2HPO4 to prepare a buffer with a pH of 7.45. Which of the following ratios of [base]/[acid] is required? For phosphoric acid, (H3PO4), Ka2 = 6.2 × 10–8. Do I use the Hasselbach equation?

2. ### chemistry

Calculate the molarity of a phosphoric acid(H3PO3)solution that is 84% by mass phosphoric acid and has a density of 1.87g/mL.?

3. ### chemistry

Use the systematic treatment of equilibrium to determine the pH and concentrations of species in 1.00 L of solution containing 0.065 mol citric acid (H3A), 0.025 mol 8-hydroxyquinoline (HB), and 0.040 mol KOH. Consider just

4. ### chemistry-help needed

How do I prepare 250ml of 10% phosphoric acid from 85% phosphoric acid(density 1.69g/ml)? Thanks.

1. ### chem

Cola contains H3PO4. In order to determine the concentration of phosphoric acid in 50.0 mL of cola, a precipitation reaction with excess silver nitrate was carried out. The product, silver phosphate, was dried and found to weigh

2. ### chemistry

Phosphoric acid reacts with magnesium carbonate to produce? When you predict the products and write a balanced equation the coefficient of phosphoric acid is a.1 b.2 c.3 d.6 e.4

3. ### Chemistry

How many resonance structures do these following acids have? H2CO3-carbonic acid H3PO4-phosphoric acid H2SO4-sulfuric acid HNO3-nitric acid CH3COOH- acetic acid CH2ClCOOH- chloroacetic acid CHCl2COOH- dichloroacetic acid CCl3COOH-

4. ### Science

Concentrated phosphoric acid is 90% by mass and the remaining mass is water.The molarity of phosphoric acid in 90% is 12.2M at rtp. a)What is the density of the solution at rtp ? b)What volume (in mL)of this solution is needed to