Using molarity to find solute mass and solution volume

Question

Calculate the volume in liters of a 0.00599 mM calcium sulfate solution that contains 25.0g of calcium sulfate . Be sure your answer has the correct number of significant digits

Answer 1

Let me walk you through this, because these are often intimidating but they're not that bad.

So you have 0.00599 mM of calcium sulfate solution. As you already know, molarity is expressed as Mol/Liter, so I'll just rewrite that number as follows: 0.00599 millimols/Liter.But I don't like working with milli in the equation, so let's change that to this: 0.00599 x 10^-3 mols/Liter.

You also know that the solution has 25 g of calcium sulfate in it. If we can figure out how many moles are present in 25 g of calcium sulfate, we can figure out the volume of the solution. We'll set up our equation so that grams cancel out & we're left with moles.

Calcium sulfate has a molar mass of 136.14 g/mol. 25 g ÷ 136.14 g/mol = 0.1836 moles.

Now we can set up our equation so that moles cancel out, & leaving us with liters:

0.00599 x 10^-3 mols/Liter. ÷ 0.1836 moles = 0.0000326 liters... or 3.26 x 10^-5 Liters.

I usually check my work with a little bit of reasoning. 25 grams of Calcium sulfate was way less than 1 mol. Our solution had a very small concentration of calcium sulfate present, so it makes sense that we got a small volume of solution in our answer.

They way I try to solve these is focusing on canceling out the right units until we're left with the appropriate units.

I hope this helps.

Warm regards,

Michael G.