Question

Calculate the molarity of a solution made by adding 39.2 mL of concentrated perchloric acid (70.5 % by mass, density 1.67 g/mL) to some water in a volumetric flask, then adding water to the mark to make exactly 2000 mL of solution. (It is important to add concentrated acid or base to water, rather than the other way, to minimize splashing and maximize safety.)

[ HClO4]= _____M

Answer 1

To study the stoichiometry in dissolutions is necessary to know the amount of the reactives that are presented in it and know how to control the used quantities of reactants to make a reaction in a aqueous dissolution.

Molarity is defined as:

M = solute moles/solution liters.

First we have to find the molar mass of HClO4

H=1.008
Cl=35.45
O=16.00*4 = 64
Molar mass = 100.458

Now that we have this we proceed to make the following conversions;

Density = mass/volume
Slcn mass= density * volume
Slcn mass= 1.67 gr/ml * 39.9gr = 66.633
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%mass HClO4 = solute mass/slcn mass *100
Solute mass = (%mass HClO4/100)*Slcn mass
Solute mass = (70.5/100) * 66.633gr
Solute mass = 46.976 gr
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Now we’re going to calculate the moles of HclO4
Moles HClO4 = Solute mass / Atomic weight
Moles HClO4 = 46.976 gr/(100.458 gr/mol) = 0.467 moles
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And finally we use the first equation above for Molarity.
M = 0.467 moles / 2 L
M = 0.2338