Help chemistry????????????

Question

when 20g of NaOH (FW 40 g/mole) is dissolved in 250 ml of water, what is the NaOH concentration of the resulting solution?

A) 0.5M

B) 1M

C) 2M

D) 4M

Help please!

Answer 1

Your unit M, stands for molarity, which is equal to Moles of a solute over Liters of solution.

First, you should convert your 20g of NaOH to moles, using your Molar mass of NaOH

20g. NaOH x 1 mol. NaOH/40g. NaOH  leaving you with .5 mol. NaOH

Now, convert your mL to L.

250 mL x 1 L/1000 mL  leaving you with .25 L

To finish, you simply find the molarity by putting moles of solute over liters of solution.

.5 mol. NaOH/.25 L = 2M

So, C) is your best answer.

Answer 2

Joe is right on.  I think of it like this, though.  If you put that 20 g into 1 L, you would have a 0.5 M solution (half a mole into a liter).  Since you have a fourth of a liter, you multiply the 0.5M by four to get 2 M.  (Same amount of solid into a fourth of the liquid...)