Consider a 0.30 M solution of phosphoric acid that is titrated with 0.100 M NaOH

ID: 513592 • Letter: C

Question

Consider a 0.30 M solution of phosphoric acid that is titrated with 0.100 M NaOH.

a) How many mL of the base would be required to convert 25.0 mL of the H3PO4 solution completely to the dihydrogen phosphate ion (H2PO4 - )

b) What volume of 0.10 M NaOH would be required to convert 25.0 mL of 0.30 M H3PO4 completely to the hydrogen phosphate ion (HPO4 2- )?

2. What is the pH of a solution made by dissolving 0.050 moles of H3PO4 and 0.050 moles of NaH2PO4 in 100 mL of water?

3. Carbonic acid, H2CO3, is a weak acid, with Ka1 = 4.3 10-7 .

a) Write the chemical equation representing the first acid dissociation reaction of carbonic acid.

b) What is the pH of a buffer solution that contains 0.50 M H2CO3 and 0.25 M HCO3 - ?

c) Write the chemical reaction that would occur if a small amount of NaOH were added to the buffer in (b)

1a) Moles of H3PO4 = (25.0 mL)*(0.30 mol/L) = 7.5 mmole.

Write down the neutralization reaction as below:

H3PO4 + NaOH ------> NaH2PO4 + H2O

As per the stoichiometric equation above,

1 mole H3PO4 = 1 mole NaOH.

Therefore, 7.5 mmole H3PO4 = 7.5 mmole NaOH.

Concentration of the supplied NaOH solution = 0.100 M.

Therefore, volume of NaOH required = (7.5 mmole)/(0.100 mol/L) = 75.0 mL (ans).

b) Write down the neutralization reaction leading to HPO42-.

H3PO4 + 2 NaOH ------> Na2HPO4 + 2 H2O

As per the stoichiometric reaction above,

1 mole H3PO4 = 2 moles NaOH.

Thus, 7.5 mmole H3PO4 = (2*7.5) mmole = 15.0 mmole NaOH.

Therefore, volume of NaOH required = (15.0 mmole)/(0.100 M) = 150.0 mL (ans).

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