Mass of NaOH needed to make 500mL of 0.1M NaOH solution?

Question

I'm working on a solutions lab and would like someone to look over my calculations please.

 

The question asked:

"What mass of solid NaOH is needed to make 500mL of 0.1M NaOH solution?"

 

My work:

Needed:  500mL solution with a final concentration of .1M

 

Na:  22.989 amu

O:   15.999 amu

H:   1.008 amu

=39.996 amu  (rounded to 40amu)

40 amu= 1 mole NaOH

 

To make one mole NaOH solution, dissolve 40 grams NaOH in 500mL of H2O.

 

mL to L conversion:

500/1000=.5

For .5L of 0.1M solution, we need 0.1M of solute.

0.1M of solute x 40 grams/Mole= 4 grams NaOH

 

The solution would be made by dissolving 4 grams NaOH in .5 Liters of H2O

Answer 1

You needed to use the molarity formula: moles of solute/Liters of solution to find how many moles of solute you needed.

You correctly converted 500 mL to 0.5 L.

Now, we can put the information we already have into the formula.

We want a solution with 0.1 M.

So, we will do 0.1=x/0.5; 0.1*0.5

Solving for x, we find that we need 0.05 moles of solute NaOH.

As you found, the molar mass of NaOH is 40 g. So, we will do 40 g*0.05, which is about 2 g.

Answer 2

Everything looks good except for this part: "To make one mole NaOH solution, dissolve 40 grams NaOH in 500mL of H2O."

To make a one molar solution you want to put one mole of the solute into 1L (1000mL) of H2O.  Then you can do your conversion the other way from L to mL.  It looks like you included to conversion here, but then didn't use it because you already had your 500mL.  

(You should end up with fewer grams of NaOH overall)