Question

A student makes a solution containing 125g of acetonitrile (CH3CN) and 300g of water. What is the mol fraction of acetonitrile?

A) 0.845

B) 0.155

C) 0.294

D) 0.706

E) 0.0453

Answer 1

Mass of acetonitrile = 125 g

Molar mass of acetonitrile = 41.05 g/mol

Moles = mass/molar mass

Moles of acetonitrile = 125g/41.05 g/mol

                                      = 3.045 mol

Mass of water = 300 g

Molar mass water = 18.01 g/mol

Moles of water = 300 g/ 18.01 g/mol

                            = 16.657 mol

Mole fraction = moles of the component / sum of moles of all components

Mole fraction of acetonitrile = moles of acetonitrile /(moles of acetonitrile + moles of water)

                                                    = 3.045 mol/(3.045 mol + 16.657mol)

                                                    = 0.155

Mole fraction of water = moles of water /(moles of acetonitrile + moles of water)

                                          = 16.657 mol /(3.045 mol + 16.657mol)

                                          = 0.845

answer : Option B