Molarity

# Molarity - Molarity and Molality Guide and Practice Written...

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Molarity and Molality Guide and Practice Written by Caleb Tatreau and John Kierstead For Wayne Wesolowski PhD -MOLARITY- Molarity can be used in a wide variety of practical applications both in a laboratory setting and in the classroom. In the most basic sense, molarity is defined as the concentration of solute in solution and can be expressed by the following equation: Molarity = liters of solvent moles of solute Units for this equation are always represented as follows: Molarity = moles / L Moles of solute = moles Liters of solution = L Application: 6.0 moles of solute are added to 1.5 liters of solution. Find the Molarity of this sample. 1.5 liters 6 moles = 4 liter moles -USING MOLARITY TO MAKE SOLUTIONS- A practical application of molarity is utilized when making solutions for use in various laboratory settings. Solutions can be diluted from their original concentration to make a lower concentration solution. The dilution can be systematically made through the use of the following equation: M 1 V 1 = M 2 V 2 M 1 =Stock (Original) Molarity V 1 =Volume of stock solution M 2 =Molarity of final (diluted) solution V 2 =Volume of final solution -MOLALITY- Molality is another representation of the concentration of a solution. However, while molarity is dependent on both volumetric measurement and mass measurement, molality is only dependent on mass measurement. Therefore, under conditions of varying temperature and pressure, the molarity will change while the molality stays the same. Molality is expressed by the following equation:

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Molality = kilograms of solvent moles of solute Units for this equation are always represented as follows: Molality = moles/kg Moles of solute = moles Kilograms of solvent = kg Application: 2.5kg of solvent are added to 4.3 moles of solute. Find the molality of this sample. 2.5 kilograms of solvent 4.3 molesof solute = 1.72 kilogram moles
Molarity Practice: 1. A solution sulfuric acid is 48% of H 2 SO 4 by mass. Calculate the number of moles in two liters of H 2 SO 4 if the density of the sulfuric acid is 1.84g/cm 3 (1cm 3 =1mL). .48 x 2000 mLx 1.84 g 98.08 g 1 mole = 18 moles Demo 18 moles of H 2 SO 4 2. Describe how you would make three liters of 2.00M acedic acid from stock 18.0M acetic acid. (3 L ) (2 M ) = ( L ) (18 M ) Demo Take 333ml of stock solution and add enough water to make three liters of solution. 3. 3.6g of KCl are dissolved into 20mL of water, what is the molarity of this solution? 3.6 gx 74.55 g 1 mole x .02 L 1 = 2.4 M Demo 2.4M of KCl 4. Muriatic acid (or HCl) is commonly added to swimming pools to adjust the pH of the water. What is the molarity of 2.4L of muriatic acid that contains 27g of HCl? 27 gx 36.45 g 1 mole x 2.4 L 1 = 0.31 M Demo 0.31M of muriatic acid 5. Ethanol is miscible in water. The concentration of alcohol can be described as proof. 200 proof is pure alcohol and a 50:50 mixture of alcohol and water is 100 proof. Calculate the molarity of 2.0L of 70 proof alcohol if the density of the ethanol is 0.894g/mL. (MW of

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## This note was uploaded on 02/05/2011 for the course CHEM 151 taught by Professor Staff during the Fall '08 term at Arizona.

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Molarity - Molarity and Molality Guide and Practice Written...

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