Question

If the concentration of formic acid (CH2O2) is 35 mg/L, how many molecules of formic acid are present in 2.0 L of this solution?

Answer 1

concentration of formic acid = 35 mg/L = 0.035 g/L

concentration of formic acid in mol/L = (g/L ) / molar mass of formic acid

                                                    = 0.035 g/L / 46 g/mol

                                                    = 0.00076 mol/L

                                                    = 0.00076 M

GIven that volume of solution = 2.0 L

We know that Molarity = moles/ Litre

                       moles = molarity x volume

                                 = 0.00076 M x 2 L

                                  = 0.00152 mol

We know that 1 moles of any species contains 6.022 x 10²³ molecules.

Therefore,

no of molecules in 0.00152 mol of formic acid = 0.00152 x 6.022 x 10²³ molecules

                                                                  = 9.15 x 10²⁰ molecules

Therefore,

molecules of formic acid present in 2.0 L of 35 mg/L solution = 9.15 x 10²⁰