Concentration Calculation Question

[Tetra]

This is a really weird question...in my opinion...

 

We are told to bake a cake, and we have to use a sulfur solution (sulfur + water ONLY). We must add 100g / L of the solution. (Yes, it's grams, not ml.) How much of the solution should we add to get that amount?

 

From what I know, concentration = mol / L. We know concentration, and we have to find the volume. What I'm not sure is how to calculate for the mol (we're not given a mass) and how to convert the g to ml?

 

I'm not sure if mass is necessary. Or, if we need one, we use a number found in society and indicate why we use it?

 

I'm not sure if I even approached this question correctly...if anyone could help, that'd be great!

[Mr Skeptic]

Yeah, put 100 grams of your solute in a 1 L container, than fill with water. If your solute is already dissolved, calculate how much is in the solution and just add enough to get 100 g solute, then fill to 1 L. (that's to get a 100 g/L solution).

 

There's two ways to measure concentration, either as moles/L or mass/L, there's also other ways but chemists hardly ever use them. To convert between the two you need to know the molecular weight (that's grams/mole). If you don't know it, just add it up from your periodic table.

[Tetra]

How about in theory? Your method was to physically add 100g of solute, then enough water to make it 1L. But what if it's just through calculations?

[Mr Skeptic]

Then you need to know how the volume of your materials change when they are dissolved in each other. If you add 1 mL of salt to 100 mL of water will you really end up with 101 mL of salt water? You won't be able to calculate it, not exactly, without knowing the density of the solution and the density of the things you add. That will get messy quickly. So, we do it without the calculation, just filling to the 1 L mark in special, very accurate flasks.