Chemistry lab question? Making a solution??? Please help!!

Question

Starting with solid sodium chloride, how would you make one liter of 2.000 ppm (parts per million) sodium chloride solution?

 

tip: 1 ppm = 1 g solute/10^6 g solution , for dilute aq solutions 1 ppm = 1 mg solute / 1 L solution

 

 

i know that 2.000 ppm = 2.000 mg/ 1 L of solution = 0.002000 g / 1 L of solution.

 

my teacher suggested to start out with an amount of 2.000 g in order to make a stock solution that is too concentrated at first because 0.002000 g is way to small to measure out. I was then told to make a precise dilution from that in order to get the answer.

 

I am really really confused and NEED help! I had never even heard of ppm before, and I am not sure where to start. All help would be greatly appreciated!!!

Answer 1

Ppm is parts per million, meaning 1 gram of solute in 1 million grams of solution, or equivalently, 1 milligram of solute to 1 liter of aqueous solution (1 L of water = 1 kg of water = 1 million milligrams of water).  Let's do as your teacher suggested and start out with the stock solution.  I've broken the problem up into parts:

 

1. Make up a stock solution.
   
   Precisely weigh out 2.000 g of solid NaCl.  Place it in a 1 L volumetric flask and add deionised water up to the mark on the flask.  Mix well.
   
   This solution has a concentration of 2.000 g/L. (Note, we often express concentrations in mol/L = molar, but g/L works better for this problem).
   
   
2. Dilute the stock solution.
   To make the required solution, we need to use a pipette to transfer a small amount of the stock solution into a 1L volumetric flask, and then add deionised water up to the mark.
   
   How much of the stock solution? Well, we need to end up with 0.002000 g of NaCl from the stock solution. To get the volume of the stock solution, you should divide this mass by the concentration 2000 g/L
   
   Here, we need to be careful with units and significant figures.
   
   
   L of stock solution = 0.002000 g ÷ (2000 g/ 1 L) = 0.002000 g × (1 L / 2000 g)  = 1.000 × 10^-6 L of stock sol.
   
   You should convert this to mL since pipettes are usually marked in mL. I'll leave it to you to do this, as well as put in the rest of the description of making up the solution.