Question

Calculate the molarity of a solution of magnesium chloride with a concentration of 44.7 mg/mL.

    a.    0.939 M

    b.    0.469 M

    c.    0.235 M

    d.    2.13 M

    e.    0.748 M

I know the answer is b, but please show me the work and label the units so I can understand :)

Answer 1

consider volume = 1 mL than mass = 44.7 mg

Molar mass of MgCl2 = 1*MM(Mg) + 2*MM(Cl)

= 1*24.31 + 2*35.45

= 95.21 g/mol

mass of MgCl2 = 44.7 mg

= 0.0447 g [using conversion 1 g = 1000 mg]

we have below equation to be used:

number of mol of MgCl2,

n = mass of MgCl2/molar mass of MgCl2

=(0.0447 g)/(95.21 g/mol)

= 4.695*10^-4 mol

volume , V = 1 mL

= 1*10^-3 L

we have below equation to be used:

Molarity,

M = number of mol / volume in L

= 4.695*10^-4/1*10^-3

= 0.469 M

Answer: 0.469 M