Question

Molar Mass question

3. Calculate the quantity, in grams, of formic acid and sodium formate necessary to make 500 mL of a buffer that has a formic acid concentration of 0.75 M and a sodium formate concentration of 0.75 M. You will need to look up or calculate the molar masses of these two compounds as your first step. See question 5 below for their chemical formulas.

Answer 1

The concentration of formic acid in the buffer solution = C = 0.75 M = 0.75 mol/L

Volume of the buffer to be made = 500 mL = 0.500 L

Hence, the number of moles of formic acid needed to make the buffer is

0.75 mol " x 0.500 L = 0.375 mol 1L

Formula of formic acid = .

нсоон

Hence, the molecular mass of formic acid can be calculated by adding the atomic masses of each element present.

m(HCOOH) = 2 x m(H) +2 x m(0) + m(C) m(HCOOH) = 2 x 1.0078 g/mol + 2 x 15.999 g/mol + 12.0107 g/mol m(HCOOH) 46.03 g/mol

Hence, the mass of formic acid needed to make the buffer is

mass = moles x molar mass = 0.375 mol x m(HCOOH) => mass = 0.375 mol x 46.03_9 17.26 g mol

Similarly, we need to have a concentration of 0.75 M for sodium formate in 500. mL of the buffer.

Hence, the number of moles of sodium formate needed is

0.75 mol " x 0.500 L = 0.375 mol 1L

Now, formula of sodium formate is .

HCOONat

Hence, the molecular mass of sodium formate can be calculated as

m(HCOONa) = 1 x m(H) + 2 x m(o) + m(C) + m(Na) = m(HCOOH) = 1 x 1.0078 g/mol + 2 x 15.999 g/mol + 12.0107 g/mol + 22.9897 g/mol > m(HCOOH) 68.01 g/mol

Hence, the mass of sodium formate needed to make the buffer can be calculated as

mass = moles x molar mass = 0.375 mol x m(HCOONa) = mass = 0.375 mol x 68.01-9~25.50 g mol

Hence, we need 17.26 g of formic acid and 25.50 g of sodium formate to make the needed quantity of buffer .