Question

A solution of ethanol (c2h5oh) in water is prepared by dissolving 75.0 ml of ethanol (density 0.79 g/cm3 ) in enough water to make 250.0 ml of solution. What is the molarity of the ethanol in this solution?

Answer 1

Given the density of ethanol = 0.79$\frac{g}{cm^{3} }$

Volume of ethanol = 75.0 mL

Calculating the mass if ethanol from density and volume:

$75.0 mL * \frac{0.79g}{1cm^{3} } *\frac{1cm^{3} }{1mL}=59.25g$

Molar mass of ethanol = (2*atomic weight of C)+(6*atomic weight of H)+(1*atomic weight of O)

                                     =(2*12g.mol) + (6*1g/mol) + (1*16g/mol)

                                     =46g/mol

Moles of ethanol = $59.25 g *\frac{1 mol}{46g} =1.288 mol$

Volume of the solution = 250.0 mL

Converting the volume from mL ot L:

$250.0mL*\frac{1L}{1000mL}=0.2500L$

Molarity of ethanol in the solution = $\frac{1.288mol}{0.250L} =5.15 M$