Writing Chemical Formula Worksheet With Answer Key - Spes Unica Page 23
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28Concentration = number of moles
Volume (in litres)
= 0.05 ÷ (50÷1000)
= 0.05 ÷ 0.05
= 1 mol/l
Example 5: How many grams of calcium chloride, CaCl
, are there in 25
2
cm 3 of a solution, concentration 0.1 mol l -1 ?
number of moles = Concentration x Volume (in litres)
= 0.1 x (25 ÷ 1000)
= 0.0025 moles
Formula mass of CaCl
= 40 + (2x 35.5)
2,
= 111g
mass of substance in grams = Number of moles x formula mass
= 0.0025 x 111
= 0.2775g
LI 13
Making a standard solution
E
A standard solution is a solution whose concentration is known
accurately. When making up a standard solution it is important that
the correct mass of substance is accurately measured. It is also
important that all of this is successfully transferred to the volumetric
flask used to make up the solution. The following procedure will make
sure that this happens.
Background calculations
1. Work out the number of moles needed to make up a solution with the
required volume and concentration.
2. Now work out the relative formula mass, of the chosen substance.
3. Work out the mass of substance needed using your answers from
steps 1 and 2.
23
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