molarity question for chem

Question

3.60 g of CaS was added to enough water to make 250 ml of solution. What is the Molarity of the solution?  

Answer 1

It helps to learn well the definition of MOLARITY. You soon will learn other types of concentrations, and it is easy to get them confused. So learning this one well now will help.

MOLARLITY is easy! It means moles per liter - or # moles you would have in ONE liter of solution. It helps to set it up as a fraction, filling in what you know.

3.60 grams CaS - I need that in MOLES. So find the formula weight of CaS (use the weights off the periodic table - and find the mass of 1 Ca plus 1 S = 72.14 grams in 1 mole). You don't have that, you only have 3.50 g. Find the moles like this:

(3.50 g)(1 mole/72.14g) = # moles.

(In chem, I set fractions up purposefully - so the units cancel and I am left ONLY with the units I want "on the top".)

MOLARLITY is MOLES PER LITER. I have moles. Now I need liters.

The volume is 250mL. I need that in LITERS.

(250mL)(1 L/1000mL) = 0.250 L

To get MOLARITY, simply put the # moles you have over the # L you have - according to the DEFINITION of MOLARITY (moles/liter) and DIVIDE to find your answer.

Good luck!