Final answer:
To determine the molar mass of an unknown acid, convert the mass of a titrant base to moles, calculate the molarity, use that to find the pH, and then use the stoichiometry of the neutralization reaction to calculate the molar mass of the acid.
Step-by-step explanation:
To find the molar mass of an unknown acid using information about a base such as NaOH, you can employ the concept of stoichiometry and neutralization reactions. Here's a step-by-step method:
- First, convert the mass of NaOH to moles using its molar mass taken from the periodic table.
- Second, calculate the molarity of the NaOH solution, as the concentration of OH- ions will be equal to the concentration of NaOH due to its complete dissociation.
- Third, use the equilibrium constant for water, Kw, to calculate the concentration of H+ ions ([H+]) in this solution.
- Fourth, convert this [H+] to pH.
- Finally, by knowing the volume and concentration (molarity) of the NaOH used to neutralize the unknown acid, you can calculate the moles of acid based on the stoichiometry of the acid-base neutralization reaction (assuming a 1:1 reaction).
- At neutrality, the moles of acid are equal to the moles of NaOH. Then, using the initial mass of the unknown acid sample, calculate its molar mass by dividing the mass by the moles of acid.
For instance, if you have a neutralization reaction of HCl(aq) with NaOH(aq) resulting in NaCl and water, using the known concentration of NaOH and the volume used at the endpoint, moles of NaOH are equal to moles of HCl. If the volume of NaOH used was 12.6 mL at a concentration of 0.100 M, this corresponds to 0.0126 L * 0.100 mol/L = 0.00126 moles. If the mass of the HCl originally present was determined to be 0.0752 g, then the molar mass of HCl would be 0.0752 g / 0.00126 mol = 59.7 g/mol.