The presence of CO3 2–, HCO3 – and CO2 in body fluids helps to stabilize the pH of these fluids desp — Acid-Base Equilibrium / Buffers Chemistry Question
Problem Context
The presence of CO3 2–, HCO3 – and CO2 in body fluids helps to stabilize the pH of these fluids despite the addition or removal of H+ ions by body processes. Answer the following questions about solutions containing these species in varying combinations at 25 °C. K1 and K2 for H2CO3 are 4.2 × 10–7 and 4.7 × 10–11, respectively.
Write balanced equations to represent the processes responsible for K1 and K2.
Model Answer
Process responsible for K1 H2CO3(aq) æ H+(aq) + HCO3 –(aq)
Process responsible for K2 HCO3 –(aq) æ H+(aq) + CO3 2–(aq)
Calculate the [H+] and pH expected for
i. 0.033 M solution of H2CO3, which is the saturation point of CO2 at 25 °C.
Model Answer
K1 = H+[ ] HCO3 –[ ] / H2CO3[ ]
4.2× 10–7 = H+[ ]2 / 0.033( )
H+[ ]= 1.2 × 10–4 pH = 3.93
ii. 1:1 mixture of H2CO3 and HCO3 –.
Model Answer
H2CO3[ ]= HCO3 –[ ] K1 = H+[ ]
H+[ ] = 4.2 ×10–7 pH = 6.38
iii. 1:1 mixture of HCO3 – and CO3 2–.
Model Answer
HCO3 –[ ]= CO3 2–[ ] K2 = H+[ ]
H+[ ] = 4.7 ×10–11 pH =10.33
iv. 0.125 M solution of CO3 2–.
Model Answer
CO3 2–(aq) + H2O(l) æ HCO3 –(aq) + OH–(aq)
OH–[ ] = Kw / Ka × CO3 2 –[ ]
OH–[ ] = 1.0× 10–14 / 4.7 ×10–11 × 0.125
OH–[ ] = 2.7× 10–5 OH–[ ]= 0.0052 pOH = 2.28
H+[ ] = 1.9× 10–12 pH = 11.72
The “normal” pH in blood plasma is 7.40. Identify the components that would provide the best buffer at this pH and calculate their ratio.
Model Answer
pH = 7.40 H+[ ] = 4.0 × 10–8 Components of the best buffer: H2CO3 and HCO3 –
4.2 × 10–7 = 4.0 × 10–8 × HCO3 – / H2CO3
HCO3 – / H2CO3 = 10.5
The value of K1 is based on the assumption that all of the CO2 dissolved in water exists in the form of H2CO3. However, recent evidence suggests that an additional equilibrium exists as represented by this equation.
CO2(aq) + H2O(l) æ H2CO3(aq)
When the “true” concentration of H2CO3(aq) is taken into account, K1 = 2 × 10–4. Use this information to determine the percent of dissolved CO2 that is actually present as H2CO3(aq).
Model Answer
4.2 × 10–7 / 2 × 10–4 = 2 × 10–3 or 0.002
Because the ratio of the two K values is 0.002, 0.2% of dissolved CO2 is actually H2CO3.