A perchloric acid solution is composed of 168.75 g of perchloric acid and 81.25 mL of pure water. What is the percent by mass of the perchloric acid solution? The density of
water is 1.000 g/mL.
When I did this part I got 67.5%
But, I am having trouble with the secind part.
The density of the above perchloric acid solution is 1.138 g/mL. What is the molarity of the perchloric acid solution?
I know molarity is moles solute/liter of solution, but how do i find that from %mass? please help if you can!! Thanks!
Use the combined mass (250 g) and the density of the solution (which you were given) to compute the volume of the solution. Once you have the mass of perchloric acid and the total volume, molarity is easily computed.
The formula for percholroic acid is HClO4. You will need that for the molecular weight, if you are computing the molarity of the solution.
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Okay, sorry....I posted it again correctly :)
To find the molarity of the perchloric acid solution, you first need to find the number of moles of perchloric acid present in the solution.
1. Calculate the mass of perchloric acid (HClO4) in the solution:
Mass of perchloric acid = 168.75 g
2. Calculate the mass of water (H2O) in the solution:
Volume of water = 81.25 mL
Density of water = 1.000 g/mL
Mass of water = Volume of water x Density of water = 81.25 g
3. Calculate the total mass of the perchloric acid solution:
Total mass = Mass of perchloric acid + Mass of water = 168.75 g + 81.25 g = 250 g
4. Calculate the percent by mass of perchloric acid in the solution:
Percent by mass = (Mass of perchloric acid / Total mass) x 100
= (168.75 g / 250 g) x 100
= 67.5%
Now, to find the molarity, we need to convert percent by mass to grams per liter and then to moles per liter.
5. Convert percent by mass to grams of perchloric acid in one liter of solution:
Percent by mass = Grams of solute / Grams of solution x 100
Grams of solute = (Percent by mass / 100) x Grams of solution
= (67.5 / 100) x 1000 g
= 675 g
6. Calculate the molarity:
Molarity = Moles of solute / Volume of solution (in liters)
Moles of solute = Grams of solute / Molar mass of solute
Molar mass of perchloric acid (HClO4) = 1 x (1.00784 g/mol) + 1 x (35.453 g/mol) + 4 x (15.999 g/mol) = 100.457 g/mol
Molarity = (675 g / 100.457 g/mol) / 1 L
= 6.72 mol/L
Therefore, the molarity of the perchloric acid solution is 6.72 mol/L.