Posted by Lori on Tuesday, December 14, 2010 at 5:38pm.


The mole fraction of an aqueous solution of potassium perchlorate is 0.125. Assuming that the density of this solution is 1.20g mL-1, calculate the following:
(A) the mass percent of potassium perchlorate in the solution.

(B) the molarity of potassium perchlorate in the solution.

chem - DrBob222, Tuesday, December 14, 2010 at 7:52pm
mole fraction KClO4 = 0.125 and mole fraction water = 0.875. If we take enough sample to have 1.00 total mol soln, we will have 0.125 moles KClO4 or 0.125 x molar mass KClO4 in grams. 0.875 moles H2O = 0.875 x molar mass water = grams water.
grams soln = grams water + grams KClO4.
mass % w/w = (mass KClO4 in grams/mass soln)*100

b)Convert g KClO4 (in previous problem) to moles. moles = grams/molar mass.
Use the density to convert g soln to L. Then M = moles/L of soln.
Post your work if you get stuck.

chem - Lori, Tuesday, December 14, 2010 at 10:52pm
Ok I got (A) to work out for me 52.4%

but i'm stuck on (B)

moles= grams/molar mass
1.20g/138.547= 8.66x10^-3 ( this cant be right)

then M = moles/L 8.66x10^-3/1.2x10^-3= 7.22

I got 7.22 but the right answer is 4.53M

but i'm stuck on (B)

moles= grams/molar mass
1.20g/138.547= 8.66x10^-3 ( this cant be right)
You are using density/molar mass to find moles which isn't correct. From part one, you already know moles = 0.125 moles KClO4 but you can calculate it again if you wish from 17.318 g KClO4/138.5486 = 0.125.
The volume of the solution from part 1 was 33.081 grams. The density is 1.20 g/mL; therefore, volume = mass/density = 33.081/1.20 = 27.5675 mL or 0.0275675 L. So M = moles/L = 0.125 moles/0.0275675L = 4.5343M which rounds to 4.53M to three s.f.


then M = moles/L 8.66x10^-3/1.2x10^-3= 7.22

I got 7.22 but the right answer is 4.53M

To calculate the mass percent of potassium perchlorate in the solution, you need to determine the masses of potassium perchlorate and the total solution.

Step 1: Calculating the mass of potassium perchlorate
Given that the mole fraction of potassium perchlorate is 0.125, it means that 0.125 moles of potassium perchlorate are present in the solution.

To find the mass of potassium perchlorate, multiply the number of moles (0.125) by the molar mass of potassium perchlorate. The molar mass of potassium perchlorate (KClO4) is calculated by summing the atomic masses of its constituent elements: K (39.10 g/mol) + Cl (35.45 g/mol) + 4O (4 x 16.00 g/mol).

So, molar mass KClO4 = 39.10 + 35.45 + (4 x 16.00) = 138.55 g/mol.

Mass of potassium perchlorate = 0.125 moles x 138.55 g/mol = 17.32 g.

Step 2: Calculating the mass of the solution
Given that the density of the solution is 1.20 g/mL, and assuming you have 1.00 mL of the solution, you can find the mass of the solution.

Mass of the solution = volume of the solution x density of the solution
Mass of the solution = 1.00 mL x 1.20 g/mL = 1.20 g.

Step 3: Calculating the mass percent
Now, you can calculate the mass percent of potassium perchlorate in the solution using the formula:

Mass percent = (mass of potassium perchlorate / mass of solution) x 100.

Mass percent = (17.32 g / 1.20 g) x 100 = 1443.33% (rounded to 2 decimal places).

Therefore, the mass percent of potassium perchlorate in the solution is approximately 1443.33%.

To calculate the molarity of potassium perchlorate in the solution, you need to consider the number of moles of potassium perchlorate and the volume of the solution.

Step 1: Calculating the moles of potassium perchlorate
Given the mass of potassium perchlorate as 17.32 g and its molar mass as 138.55 g/mol, you can calculate the number of moles.

Moles of potassium perchlorate = mass of potassium perchlorate / molar mass
Moles of potassium perchlorate = 17.32 g / 138.55 g/mol = 0.125 mol.

Step 2: Calculating the molarity
Given the density of the solution as 1.20 g/mL, and assuming you have 1.00 mL of the solution, you can calculate the volume of the solution.

Volume of the solution = 1.00 mL (given).

Now, you can calculate the molarity using the formula:

Molarity (M) = Moles of solute / Volume of solution (in liters).

Molarity = 0.125 mol / (1.00 mL รท 1000 mL/L) = 0.125 mol / 0.001 L = 125 mol/L.

Therefore, the molarity of potassium perchlorate in the solution is 125 mol/L.

In your calculations, it seems you made an error when converting grams to moles. The correct answer is obtained by dividing the mass (1.20 g) by the molar mass (138.55 g/mol), giving a value of approximately 0.00866 moles, not 8.66 x 10^(-3) moles. Then, when dividing by 1.20 x 10^(-3) L, you should obtain a value of approximately 7.22 M, not 4.53 M.