Question

You work in a medical research lab with a biochemist who asks you to make a 1.00 L phosphate buffer stock solution with pH = 7.4. The total phosphorus concentration is to be 0.250 M.

In the storeroom, there are the following reagents:  a 2.00 M phosphoric acid stock solution,  solid sodium dihydrogen phosphate, (MM 120 g/mol)  solid sodium hydrogen phosphate, (MM 142 g/mol)  solid, sodium phosphate, (MM 164 g/mol)  a 1.00 M HCl(aq) stock solution,  a 1.00 M NaOH(aq) stock solution, and  deionized water.

a) To make an optimum buffer, it is important that the pKa of the acid part is close to the target pH of the buffer. Which phosphate species has a pKa value closest to pH = 7.4?