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Molarity & Dilution Aleigha Benoit Chemistry 12 December 15th

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Helpful Definitions Solutions: homogeneous mixture of two or more substances physically mixed together in a uniform way. Solute: substance being dissolved. Solvent: part of a solution doing the dissolving. Soluble: when a substance dissolves in another substance. Solubility: the ability of a substance to dissolve in another substance. Dilute: more solvent than solute in a solution.

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Molarity Molarity (also called concentration) is a measure of the amount of solute that is dissolved in a given amount of solution. Molarity is the number of moles of solute dissolved in one litre of solution. Therefore, the equation to find the molarity (M) of a solution is : M= n/v = moles of solute / litres of solution. The units are moles per litre. The simplified version for the equation M=n/v is M=g/mm x L, g meaning grams, mm meaning the molar mass of the solution, and L meaning litres. Be careful not to get molarity and moles mixed up. Moles measures the amount of material you have. Molarity measures the concentration of the material.

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Molarity Example #1 Reminder: To make the question easier to work with, convert any unit of mass to grams and any unit of volume to litres. 1. Calculate the molarity of a 5L solution containing 126g of HNO 3. Calculate the number of moles: 126g HNO 3 ______ =2 moles M= moles of solute liters of solution M= 2mol HNO3 5L M= 0.4 mol/L

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Molarity Example #2 Calculate the mass of NaOH needed to prepare 1.0L of a 1.5M solution. ( To do this, rearrange the equation M= g/mm x L to g= mm x m x L.) What we need to find out: ___g? What we know: Molar Mass (mm)= 40g/mol Molarity : 1.5M Liters: 1L Plug these values into the equation : g= mm x m x L g= (40g/mol)(1.5mol/L)(1L) g= 60g

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Molarity Example #3 960g of NaOH is used in preparing a 1.5M solution. What volume of solution can be made? (Rearrange M=g/mm x L to L= g/mm x m) What we need to find out: ___L? What we know: g= 960 mm= 40g/mol (Periodic Table) M= 1.5mol/L Plug these values into the equation: L= g/mm x m L= (960g)(40g/mol)(1.5mol/L) L= 16L

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Dilution Dilution is the process of decreasing the concentration of a stock solution by adding more solvent to the solution. The solvent added is usually the universal solvent, known as water. The more solvent you add, the more diluted the solution will get. A stock solution is a concentrated solution that will be diluted to a lower concentration for actual use. The equation for dilution is M 1 V 1 =M 2 V 2 stock solution= diluted solution M 1 = molarity of the stock solution M 2 = molarity of the diluted solution V 1 = volume of stock solution V 2 = volume of diluted solution

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The Dilution Equation The equation for dilution is M 1 V 1 =M 2 V 2 stock solution= diluted solution M 1 = molarity of the stock solution M 2 = molarity of the diluted solution V 1 = volume of stock solution V 2 = volume of diluted solution Basically, dilution calculations involve figuring out the final concentration or volume ( depending on what’s given and what’s known) after a volume or concentration has been changed. In dilution equations, you are given three things and you need to find the forth component.

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Dilution Example #1 A stock solution of 1.00M of NaCl is available. How many milliliters are needed to make a mL of 0.750M? What we know: the molarity of the stock solution which is 1.00M, and the two components of the diluted solution which are M 2 = 0.750M and V 2 = 100 mL. Plug in the values you have into the equation to solve for the missing value. M 1 V 1 =M 2 V 2

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Dilution Example #2 Concentrated HCl is 12M. What volume is needed to make 2L of a 1M solution? What we know: the molarity of the stock solution which is 12M, and the two values for the diluted solution which are M 2 =1M and V 2 =2L. Plug in the values you have into the equation to solve for the missing value. M 1 V 1 =M 2 V 2

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Dilution Example #3 Calculate the final concentration if 2L of 3M of NaCl and 4L of 1.50M of NaCl are mixed. Assume there is no volume contraction upon mixing. For this, you must use the equation M= total mol/ total volume. So you must find the total number of moles to do this calculation.

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