Calculate the moles of NaCl in the following aliquot...

Question

a solution was prepared adding 23.512g of NaCl to enough water to obtain 250.00mL of solution. A 10.0mL aliquot of the solution was taken. Calculate the moles of NaCl in the aliquot. 

Answer 1

First off lets calculate the moles in the original solution.

 

What is the molar mass of sodium chloride?  Na=23.0 and Cl=35.5 (I am choosing 3 sf as that is the periodic table I have on hand but 23.512g is 5SF) MM=58.5 (3SF but you should do 5SF)

 

Mass/Molar mass = moles 23.512g/58.5gmol-1=0.402 moles (3SF)

 

Mol X Vol=moles.  Lets check what we have.  We know the moles and we know the volume 250.00 mL therefore rearranging our equation Mol=moles/vol (note molarity is moles per liter so for dimensional analysis volume needs to be 0.250L)

 

0.402 moles/0.250L=1.61M (3SF remember you could be doing this to 5)

 

Now we are going to take 10mL of our 1.61 M solution.

We will use the equation

Mol X Vol = moles for our final answer

Vol needs to be in litres so we will divide 10 by 1000 to get 0.01L  (note 10.0mL was 3 SF so now our answer really should be 3SF)

 

1.61 M (moles/L) X 0.01L = 0.0161 moles

 

Hope this all makes sense.