6. An environmental chemist needs a carbonate buffer of pH 10.00 to study the effects of acid rain on limestone-rich soils. To prepare this buffer she will add solid Na2CO3 (106.0 g/mol) to 1.50 L of 0.20 M NaHCO3. (Ka = 4.7 x 10-11) HCO3 - (aq) + H2O (l) ↔ CO3 2 - (aq) + H3O + (aq) A. What must the final concentration of Na2CO3 be once mixed with the NaHCO3 solution to obtain a buffer with the correct pH?

Question

Chemistry

Naomi Krause

11 February, 08:26

6. An environmental chemist needs a carbonate buffer of pH 10.00 to study the effects of acid rain on limestone-rich soils. To prepare this buffer she will add solid Na2CO3 (106.0 g/mol) to 1.50 L of 0.20 M NaHCO3. (Ka = 4.7 x 10-11) HCO3 - (aq) + H2O (l) ↔ CO3 2 - (aq) + H3O + (aq) A. What must the final concentration of Na2CO3 be once mixed with the NaHCO3 solution to obtain a buffer with the correct pH?

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Answers (1)

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Judith Browning · Answer 1

[Na₂CO₃] = 0.094M

Explanation:

Based on the reaction:

HCO₃⁻ (aq) + H₂O (l) ↔ CO₃²⁻ (aq) + H₃O⁺ (aq)

It is possible to find pH using Henderson-Hasselbalch formula:

pH = pka + log₁₀ [A⁻] / [HA]

Where [A⁻] is concentration of conjugate base, [CO₃²⁻] = [Na₂CO₃] and [HA] is concentration of weak acid, [NaHCO₃] = 0.20M.

pH is desire pH and pKa (10.00) is - log pka = - log 4.7x10⁻¹¹ = 10.33

Replacing these values:

10.00 = 10.33 + log₁₀ [Na₂CO₃] / [0.20]

[Na₂CO₃] = 0.094M