Question

Answer the following and express your answer using two significant figures. Calculate the grams of solute needed to prepare each of the following solutions.

A) 1.6 L of a 4.5 M NaOH solution

B) 7.0 L of a 0.20 M CaCl2 solution

C) 180 mL of a 3.50 M NaNO3 solution

Answer 1

A) Molarity of a solution = no.of moles of solute/volume of solution in litres

                                        M = n/V (mol/L)

                                      4.5M = n/ 1.6L

n= 4.5M * 1.6L   

        = 7.2 moles

The amount of NaOH in 7.2 moles = (no.of moles) 7.2 * molecular mass of NaOH

Molecular mass of NaOH = 40 g/mol

The amount of NaOH in 7.2 moles = 7.2 mol* 40 g/mol

                                                        = 288g

                                                        = 290g (two significant figures)

B) 7.0 L of a 0.20 M CaCl2 solution

Molarity of a solution = no.of moles of solute/volume of solution in litres
                                        M = n/V (mol/L)

                                     0.2M = n / 7.0 L

n = 0.2M * 7.0 L

         = 1.4 moles

The amount of CaCl2 in 1.4 moles = (no.of moles) 1.4 * molecular mass of CaCl2

                                                       = 1.4 mol * 111 g/mol

                                                       = 155.4g

                                                       = 160g (to two significant figures)

C) 180 mL of a 3.50 M NaNO3 solution:

Molarity of a solution = no.of moles of solute/volume of solution in litres
                                        M = n/V (mol/L)

                                       3.5M = n / 0.18L

n = 0.63 moles

The amount of NaNO3 in 0.63 moles = (no.of moles) 0.63 * molecular mass of NaNO3

                                                       = 0.63 mol * 85 g/mol

                                                      = 53.55 g

                                                      = 54 g ( to two significant figures)