Friday, 30 December 2011

MOLARITY!

Hi everyone! So last class we learned about the molar concentration aka molarity of solutions.

Recall:
- solution = a homogenous mixture where one substance is dissolved in another
- solvent = larger quantity; dissolves the solute
- solute = smaller quanitity; it's the amount of solute being dissolved in a certain amount of solution



Why do we need to know molarity?
ANSWER: To compare the amount of solute dissolved in a certain volume of solution!

"Molar Concentration" (aka molarity) is the number of moles of a solute in 1 litre of solution

  • measures the concentration of solute in the solution
  • "M" is the symbol we use to represent molarity
  • "mol/L" is the unit


There are 3 different formulas (but as long as you know one, you can find the others):

1) MOLARITY = Moles/Litres
2) LITRES = Moles/ Molarity
3) MOLES = Molarity x Litres


Let's see some examples! 

1) Calculate the MOLARITY of a 1.5 L solution that contains 0.24 moles of HCL.

    Formula: M = mol/L
                  M = 0.24mol/1.5L
                  M = 0.16M of HCL

2) Calculate the concentration of 60 mL solution that contains 0.075 moles of NH4Cl. 

    Formulas: M = mol/L
                   M = 0.075mol/(0.060L)
                   M = 1 M  

3) What is the mass of solute for 125mL of 0.25 M Ba(NO3)2?

    Formula: mol = L x M
                  mol = 125mL x 1L/1000mL = 0.125 L
                  mol = 0.125L x 0.25 M
                  mol = 0.03125 mol Ba(No3)2 
                  0.031 mol x 261.3g of Ba(NO3)2/ 1mol = 8.2g of Ba(NO3)2 

 4) What volume in ml of a 0.25 mol/L NaOH solution is needed for 0.050mol of NaOH?


   Formula: L = mol/M
                 L = 0.050mol/0.25M
                 L = 0.20 L x 1000 = 2.0 x 10^2 mL



Here is an informative video on molarity to further educate you guys!




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