Buffer / pH: practice problems Work out the conjugate base and weak acid concentrations. Every answer here is computed by the same tested calculator the rest of the site uses, so a solution can never drift away from the maths.
Problem 1 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 8.06 Target pH: 8 Total concentration: 40 mM Show the answer and the working For pH 8, combine 18.62 mM conjugate base with 21.38 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(8 − 8.06) = 0.871 Base fraction: r / (1 + r) = 0.4655 Acid fraction: 1 / (1 + r) = 0.5345 Conjugate base [A⁻]: C_total · r/(1+r) = 40 mM · 0.4655 = 18.62 mM Weak acid [HA]: C_total · 1/(1+r) = 40 mM · 0.5345 = 21.38 mM Open this in the calculator to change the numbers.
Problem 2 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 4.76 Target pH: 5.76 Total concentration: 100 mM Show the answer and the working For pH 5.76, combine 90.91 mM conjugate base with 9.091 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(5.76 − 4.76) = 10 Base fraction: r / (1 + r) = 0.9091 Acid fraction: 1 / (1 + r) = 0.09091 Conjugate base [A⁻]: C_total · r/(1+r) = 100 mM · 0.9091 = 90.91 mM Weak acid [HA]: C_total · 1/(1+r) = 100 mM · 0.09091 = 9.091 mM Open this in the calculator to change the numbers.
Problem 3 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 8.06 Target pH: 8.5 Total concentration: 50 mM Show the answer and the working For pH 8.5, combine 36.68 mM conjugate base with 13.32 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(8.5 − 8.06) = 2.754 Base fraction: r / (1 + r) = 0.7336 Acid fraction: 1 / (1 + r) = 0.2664 Conjugate base [A⁻]: C_total · r/(1+r) = 50 mM · 0.7336 = 36.68 mM Weak acid [HA]: C_total · 1/(1+r) = 50 mM · 0.2664 = 13.32 mM Open this in the calculator to change the numbers.
Problem 4 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 4.76 Target pH: 4.76 Total concentration: 100 mM Show the answer and the working For pH 4.76, combine 50 mM conjugate base with 50 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(4.76 − 4.76) = 1 Base fraction: r / (1 + r) = 0.5 Acid fraction: 1 / (1 + r) = 0.5 Conjugate base [A⁻]: C_total · r/(1+r) = 100 mM · 0.5 = 50 mM Weak acid [HA]: C_total · 1/(1+r) = 100 mM · 0.5 = 50 mM Open this in the calculator to change the numbers.
Problem 5 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 8.06 Target pH: 8 Total concentration: 50 mM Show the answer and the working For pH 8, combine 23.28 mM conjugate base with 26.72 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(8 − 8.06) = 0.871 Base fraction: r / (1 + r) = 0.4655 Acid fraction: 1 / (1 + r) = 0.5345 Conjugate base [A⁻]: C_total · r/(1+r) = 50 mM · 0.4655 = 23.28 mM Weak acid [HA]: C_total · 1/(1+r) = 50 mM · 0.5345 = 26.72 mM Open this in the calculator to change the numbers.
Problem 6 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 4.76 Target pH: 4.76 Total concentration: 250 mM Show the answer and the working For pH 4.76, combine 125 mM conjugate base with 125 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(4.76 − 4.76) = 1 Base fraction: r / (1 + r) = 0.5 Acid fraction: 1 / (1 + r) = 0.5 Conjugate base [A⁻]: C_total · r/(1+r) = 250 mM · 0.5 = 125 mM Weak acid [HA]: C_total · 1/(1+r) = 250 mM · 0.5 = 125 mM Open this in the calculator to change the numbers.
Problem 7 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 8.06 Target pH: 8 Total concentration: 20 mM Show the answer and the working For pH 8, combine 9.31 mM conjugate base with 10.69 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(8 − 8.06) = 0.871 Base fraction: r / (1 + r) = 0.4655 Acid fraction: 1 / (1 + r) = 0.5345 Conjugate base [A⁻]: C_total · r/(1+r) = 20 mM · 0.4655 = 9.31 mM Weak acid [HA]: C_total · 1/(1+r) = 20 mM · 0.5345 = 10.69 mM Open this in the calculator to change the numbers.
Problem 8 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 8.06 Target pH: 7 Total concentration: 20 mM Show the answer and the working For pH 7, combine 1.602 mM conjugate base with 18.4 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(7 − 8.06) = 0.0871 Base fraction: r / (1 + r) = 0.08012 Acid fraction: 1 / (1 + r) = 0.9199 Conjugate base [A⁻]: C_total · r/(1+r) = 20 mM · 0.08012 = 1.602 mM Weak acid [HA]: C_total · 1/(1+r) = 20 mM · 0.9199 = 18.4 mM Open this in the calculator to change the numbers.
Problem 9 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 4.76 Target pH: 4.26 Total concentration: 200 mM Show the answer and the working For pH 4.26, combine 48.05 mM conjugate base with 151.9 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(4.26 − 4.76) = 0.3162 Base fraction: r / (1 + r) = 0.2403 Acid fraction: 1 / (1 + r) = 0.7597 Conjugate base [A⁻]: C_total · r/(1+r) = 200 mM · 0.2403 = 48.05 mM Weak acid [HA]: C_total · 1/(1+r) = 200 mM · 0.7597 = 151.9 mM Open this in the calculator to change the numbers.
Problem 10 Buffer / pH Work out the conjugate base and weak acid concentrations.
pKa: 8.06 Target pH: 8.5 Total concentration: 30 mM Show the answer and the working For pH 8.5, combine 22.01 mM conjugate base with 7.991 mM acid.
Base : acid ratio: r = [A⁻]/[HA] = 10^(pH − pKa) Substitute: r = 10^(8.5 − 8.06) = 2.754 Base fraction: r / (1 + r) = 0.7336 Acid fraction: 1 / (1 + r) = 0.2664 Conjugate base [A⁻]: C_total · r/(1+r) = 30 mM · 0.7336 = 22.01 mM Weak acid [HA]: C_total · 1/(1+r) = 30 mM · 0.2664 = 7.991 mM Open this in the calculator to change the numbers.
More buffers practice
Same topic, different calculation Dial in the bench pH so your buffer hits target at working temperature.
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Open the Buffer / pH calculator