MOLE CALCULATIONS REVISION

THIS IS A SUMMARY TO THE MOLE CALCULATIONS,ALMOST ALL MOLE CALCULATIONS...THESE HELPED ME ALOT,,HOPE THEY HELP U TOO..

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  • Created on: 05-05-13 15:19
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Chemistry: Topic 1.1 Calculations
Calculating No. of Moles:
There are 3 ways to calculate the number of moles (depending on the information given):
1. Moles (of a substance) = given mass in grams/Ar or Mr
2. Moles (in a solution) = concentration x (volume in cm³/1000)
3. Moles (of a gas) = volume in cm³/24000cm³
Empirical Formula
Gives you the moles of each atom and the ratio of moles
1. Divide given percentage/mass by r.a.m
2. Divide each answer by the smallest answer
3. If ratios are not whole numbers, multiply each answer by a small integer
Example:
A compound contains 39.13% carbon, 52.17% oxygen and 8.70% hydrogen. Calculate
the empirical formula.
Step 1 (divide by r.a.m)
Carbon: 39.13/12 = 3.26
Oxygen: 52.17/16 = 3.25
Hydrogen: 8.70/1 = 8.70
Step 2 (divide by smallest)
Carbon: 3.26/3.25 = 1
Oxygen: 3.25/3.25 = 1
Hydrogen: 8.70/3.26 = 2.66
Step 3 (multiply all by small integer)
Carbon: 1 x 3 = 3
Oxygen: 1 x 3 = 3
Hydrogen: 2.66 x 3 = 8
.:. Empirical formula is C3H8O3
Molecular Formula
To find the molecular formula we need to use the molecular formula mass and the
empirical formula mass.
Molecular formula mass = empirical formula mass x an integer
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Example:
A hydrocarbon with Relative Molecular Mass (Mr) of 28gmol-1 has the following
composition: carbon 85.7% and hydrogen 14.3%. Calculate the hydrocarbon's molecular
formula.
Step 1 (find empirical formula mass)
Carbon: 85.7/12 = 7
Hydrogen: 14.3/1 = 14.3
Carbon: 7/7 = 1
Hydrogen: 14.3/7 = 2
.:. Empirical formula = CH2
.:.…read more

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Example:
What is the volume of 5.6 x 10-2 moles of chlorine molecules?
Volume = 0.0056 x 24000 = 134.4cm³
Example:
Calculate the volume of 10g of hydrogen gas.
Moles = 10/2 = 5 moles
Volume = 5 x 24000 = 120000cm³ = 120dm³
Example:
Calculate the mass of 1000cm³ of carbon dioxide.
Moles = 1000/24000 = 4.17 x 10 -2
Mass = 0.0417 x 44
= 1.8g
Volumetric Analysis
Calculations involving solutions.…read more

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