Chemistry 20 Solution Concentration Molarity

Transcription

1 Chemistry 20 Solution Concentration Molarity The standard unit of concentration in chemistry is molar concentration (mol/l). It is also known as molarity. This is the amount of a chemical entity, in moles, dissolved in 1 litre of solvent. A mole is a unit used to represent a specific number of items, just like other units: Unit # of items 1 dozen 12 1 ream mole 6.02 x x is known as Avogadro s number. An element s atomic molar mass on the periodic table is the mass of one mole (6.02 x atoms) of that element. The molar mass of a chemical is the sum of all the atomic molar masses in the chemical formala: Recall from Science 10 that the molar mass of a chemical can be calculated using the chemical formula and the periodic table: e.g. Sr 3 (PO 4 ) 2 Sr 3 x = P 2 x = O 8 x = g/mol!! The balancing numbers in a chemical equation represent the moles of that chemical entity reacting or being produced. 2 H 2 (g) + O 2 (g) 2 H 2 O (l) 2 moles 1 mole 2 moles hydrogen oxygen water Chemistry 20 Lesson 3-03 Page 1

2 The amount of moles of an entity (chemical) can be calculated if we know the mass involved and the molar mass of the entity. Formula for calculating moles (from Science 10): m n = M Where: n = amount of moles (mol) m = mass of entity (g) M = molar mass of entity (g/mol) Example question: What amount of moles of strontium phosphate is present in 3.57 g? = = / n = mol The concentration of a chemical solution is the ratio of the amount (moles) of solute per litre of solvent. This is known as molar concentration or molarity. Formula for molar concentration: Example 1 = Where: C = concentration in mol/l n = amount of solute in moles (mol) V = volume of solution in L Note: this formula is almost always used with the mass-to-moles formula Determine the concentration of a solution made by dissolving 4.57 g of strontium phosphate in 750 ml of water. Variables: C =? m = 4.57g strontium phosphate: Sr 3(PO 4) 2 M = g/mol V = L First find n from the mass: Now find C = = n = mol = = C = mol/l Chemistry 20 Lesson 3-03 Page 2

3 Example 2 Determine the mass of yttrium nitrate required to make 2.50 L of a mol/l solution. Variables: C = mol/l m =? yttrium nitrate: Y(NO 3) 3 M = g/mol V = 2.50 L In order to find mass you first need to find moles n = CV n = mol/l x 2.50 L = mol now find mass m = nm m = mol x g/mol m = 133 g Chemistry 20 Lesson 3-03 Page 3

4 Chemistry 20 Formative Problem Set Molar Concentration (Molarity) Moles formula: m n= M Molar concentration (molarity) formula: Concentration from mass and volume n C= V 1. A sample of household ammonia contains 156 g of NH 3 (g) dissolved in water to form 2.00 L of solution. Calculate the molar concentration of the household ammonia solution. 2. When 11.0 g of glacial (pure) acetic acid is dissolved in water to make 250 ml of vinegar solution, what is the molar concentration of the vinegar? 3. Determine the molar concentration of 500 ml of a solution that contains 12.7 g of swimming pool chlorinator, calcium hypochlorite. 4. A solution for waterproofing concrete may be prepared by dissolving 200 g of ammonium stearate (NH 4 C 17 H 35 COO) in water to make 5.00 L of solution. Determine the molarity of the solution. 5. A car battery terminal protective coating can be prepared by dissolving g of sodium silicate in 250 ml of water. Calculate the molarity of the solution. Volume from mass and concentration 1. Chlorine bleach in its solution form usually is sold as a 5 to 6 percent solution of sodium hypochlorite (e.g. Chlorox and Purex). How many litres of a mol/l solution would contain g of the chemical? 2. A solution of sodium phosphate makes a very efficient cleaner for old paintbrushes. What volume of mol/l brush cleaning solution can be prepared from 126 g of sodium phosphate? 3. Painting the terminals with a protective coating of concentrated sodium silicate can prevent corrosion of battery terminals. What volume of 8.20 mol/l coating solution can be prepared from g of sodium silicate? 4. Pots, kettles, frying pans and other nonaluminum household utensils can be cleaned of grease and baked-on foods by using a lye (sodium hydroxide) solution. If using 10.0 g of sodium hydroxide to prepare a mol/l solution, what is the volume of water required? Mass from volume and concentration 1. A toilet bowl cleaner may be prepared by mixing sodium hydrogen carbonate (baking soda) and sodium hydroxide (lye). What mass of sodium hydrogen carbonate must be added to a 2.50 L bowl to obtain a necessary mol/l solution? 2. What mass of sodium hydroxide must be added to the toilet bowl, along with the baking soda in question 1, if the molarity of the lye must be mol/l? 3. Sodium silicate is very soluble in water and its concentrated solution is sold commercially under the name water glass. Sodium silicate may be used with sodium phosphate and sodium metaphosphate to prepare a water softener for dishes and laundy. What mass of sodium silicate is necessary to prepare 10.0 L of a mol/l water-softening solution? 4. Sodium thiosulphate pentahydrate is known in photography as hypo and is used as a fixer because it readily dissolves silver compounds. Calculate the mass of hypo needed to prepare 100 ml of a mol/l solution? Chemistry 20 Lesson 3-03 Page 4

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