Methanoic acid HCO2H(aq) also known as formic acid, is partly responsible for the characterisitic itchy rash produced by the leaves of the stinging nettle plant. Calculate the pH of 0.150 mol/L methanoic acid. The Ka for methanoic acid is 1.8 x 10^-4.
repeat. Follow the lactic acid procedure.
To calculate the pH of a solution of methanoic acid, you need to use the equilibrium constant expression for its dissociation reaction and solve the resulting quadratic equation. Here are the steps to follow:
Step 1: Write the balanced equation for the dissociation of methanoic acid (HCO2H) in water:
HCO2H ⇌ H+ + HCO2-
Step 2: Write the expression for the equilibrium constant (Ka):
Ka = [H+][HCO2-] / [HCO2H]
Step 3: Write the initial and equilibrium concentrations of the species involved. Since the initial concentration of methanoic acid is 0.150 mol/L and it dissociates completely, the initial concentration of HCO2H is also 0.150 mol/L, while the initial concentrations of H+ and HCO2- are both zero.
Step 4: Express the equilibrium concentrations of H+ and HCO2- in terms of the degree of dissociation, represented by 'x':
[H+] = x
[HCO2-] = x
Step 5: Express the equilibrium concentration of HCO2H in terms of the degree of dissociation, subtracting 'x' from the initial concentration:
[HCO2H] = 0.150 - x
Step 6: Substitute these expressions into the equilibrium constant expression (Ka) to obtain the quadratic equation:
Ka = x * x / (0.150 - x)
Step 7: Apply the value of Ka (1.8 x 10^-4) and solve for 'x' using the quadratic equation solver. The quadratic equation you obtained in Step 6 will give you two possible values for 'x'. However, since the concentration of H+ is always smaller than the concentration of methanoic acid, you can discard the larger value as unphysical. Therefore, only the smaller value of 'x' will be used for further calculations.
Step 8: Once you have the value of 'x,' substitute it into [H+] = x and calculate the pH using the formula:
pH = -log[H+]
Following these steps, you should be able to calculate the pH of the methanoic acid solution.