Hello, Janet,
Please review my answer to your question concerning moles in an NaOH solution. Start with the defiunition of Molar: moles/liter.
Calculate the molarity of 0.300 mol of Na2S in 1.95 L of solution.
We have 0.300 moles of Na2S in 1.95 liters of solution. Therefore,
0.300 moles/1.95 liters = 0.154 M with 3 sig figs.
Calculate the molarity of 20.1 g of MgS in 865 mL of solution.
The molar mass of MgS is 56.37 g/mole. 20.1 grams is (20.1 grams)/(56.37 grams/mole) = 0.357 moles MgS.
This is dissolved to make 865 ml, or 0.865 L of soultion.
(0.357 moles/0.865 L) = 0.412 M (3 sig figs)
Bob
