chemistry 20 solution concentration –...

Chemistry 20 Lesson 3-03 © 2007 T. de Bruin

Chemistry 20 Solution Concentration – Molarity

The standard unit of concentration in chemistry is molar concentration (mol/L). It is also known asmolarity. 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 items1 dozen 121 ream 5001 mole 6.02 x 1023

· 6.02 x 1023 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 1023 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 chemicalformula and the periodic table:

e.g. Sr3(PO4)2 Sr 3 x 87.62 = 262.86 P 2 x 30.97 = 61.94O 8 x 16.00 = 128.00

452.80 g/mol!

! The balancing numbers in a chemical equation represent the moles of that chemical entity reactingor being produced.

· 2 H2 (g) + O2 (g) ® 2 H2O (l)2 moles 1 mole 2 moleshydrogen oxygen water


mnM

=

· The amount of moles of an entity (chemical) can be calculated if we know the mass involved andthe molar mass of the entity.

Formula for calculating moles (from Science 10):

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?

= = 3.57

452.80 /n = 0.00788 mol

· The concentration of a chemical solution is the ratio of the amount (moles) of solute per litre ofsolvent.

· This is known as molar concentration or molarity.

Formula for molar concentration:

=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

Example 1

Determine the concentration of a solution made by dissolving 4.57 g of strontium phosphate in 750 mLof water.

Variables:C=?m=4.57gstrontiumphosphate:

Sr3(PO4)2M=452.80g/molV=0.750L

Firstfindnfromthemass:

= = 4.57

452.80

n=0.0100927562mol

NowfindC

= = 0.0100927562

0.750

C=0.0135mol/L


Example 2

Determine the mass of yttrium nitrate required to make 2.50 L of a 0.193 mol/L solution.

Variables:C=0.193mol/Lm=?yttriumnitrate:Y(NO3)3M=274.94g/molV=2.50L

Inordertofindmassyoufirstneedtofindmolesn=CVn=0.193mol/Lx2.50L=0.4825mol

nowfindmassm=nMm=0.4825molx274.94g/molm=133g


Chemistry 20 Formative Problem Set – Molar Concentration (Molarity)

Moles formula:m

n=M

Molar concentration (molarity) formula:n

C=V

Concentration from mass and volume

1. A sample of household ammonia contains 156 gof NH3 (g) dissolved in water to form 2.00 L ofsolution. Calculate the molar concentration ofthe household ammonia solution.

2. When 11.0 g of glacial (pure) acetic acid isdissolved in water to make 250 mL of vinegarsolution, what is the molar concentration of thevinegar?

3. Determine the molar concentration of 500 mLof a solution that contains 12.7 g of swimmingpool chlorinator, calcium hypochlorite.

4. A solution for waterproofing concrete may beprepared by dissolving 200 g of ammoniumstearate (NH4C17H35COO) in water to make 5.00L of solution. Determine the molarity of thesolution.

5. A car battery terminal protective coating can beprepared by dissolving 240.0 g of sodiumsilicate in 250 mL of water. Calculate themolarity of the solution.

Volume from mass and concentration

1. Chlorine bleach in its solution form usually issold as a 5 to 6 percent solution of sodiumhypochlorite (e.g. Chlorox and Purex). Howmany litres of a 0.800 mol/L solution wouldcontain 119.2 g of the chemical?

2. A solution of sodium phosphate makes a veryefficient cleaner for old paintbrushes. Whatvolume of 0.700 mol/L brush cleaning solutioncan be prepared from 126 g of sodiumphosphate?

3. Painting the terminals with a protective coatingof concentrated sodium silicate can preventcorrosion of battery terminals. What volume of8.20 mol/L coating solution can be preparedfrom 240.0 g of sodium silicate?

4. Pots, kettles, frying pans and other non-aluminum household utensils can be cleaned ofgrease and baked-on foods by using a lye(sodium hydroxide) solution. If using 10.0 g ofsodium hydroxide to prepare a 0.100 mol/Lsolution, what is the volume of water required?

Mass from volume and concentration

1. A toilet bowl cleaner may be prepared bymixing sodium hydrogen carbonate (bakingsoda) and sodium hydroxide (lye). What massof sodium hydrogen carbonate must be addedto a 2.50 L bowl to obtain a necessary 0.150mol/L solution?

2. What mass of sodium hydroxide must be addedto the toilet bowl, along with the baking soda inquestion 1, if the molarity of the lye must be0.075 mol/L?

3. Sodium silicate is very soluble in water and itsconcentrated solution is sold commerciallyunder the name water glass. Sodium silicatemay be used with sodium phosphate andsodium metaphosphate to prepare a watersoftener for dishes and laundy. What mass ofsodium silicate is necessary to prepare 10.0 L ofa 0.00500 mol/L water-softening solution?

4. Sodium thiosulphate pentahydrate is known inphotography as hypo and is used as a fixerbecause it readily dissolves silver compounds.Calculate the mass of hypo needed to prepare100 mL of a 0.120 mol/L solution?

chemistry 20 solution concentration –...

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