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This document provides an introduction to measuring amounts of substances in chemistry. It discusses converting between mass and moles using molar mass, and between volume, moles, and concentration for solutions and gases. Formulas are given to calculate moles from mass or volume measurements using molar mass, molar concentration, or the gas law PV=nRT. Examples problems are provided applying these formulas to calculate values like moles, mass, volume, or concentration given other values. The document marks the end of Part 1 on measuring amounts of substances.

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61 views6 pages

As Transition Pack

This document provides an introduction to measuring amounts of substances in chemistry. It discusses converting between mass and moles using molar mass, and between volume, moles, and concentration for solutions and gases. Formulas are given to calculate moles from mass or volume measurements using molar mass, molar concentration, or the gas law PV=nRT. Examples problems are provided applying these formulas to calculate values like moles, mass, volume, or concentration given other values. The document marks the end of Part 1 on measuring amounts of substances.

Uploaded by

Elastic Fantastic
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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AS TRANSITION COURSE

SUMMER 2016
PART 1: MEASURING AMOUNT OF
SUBSTANCE

MASS VOLUME MOLAR MASS AVOGADRO

CONCENTRATION ATOM ION MOLECULE


MEASUREMENTS IN CHEMISTRY

Mass
Convert the following into grams:

a) 0.25 kg
b) 15 kg
c) 100 tonnes
d) 2 tonnes

Volume
Convert the following into dm3:

a) 100 cm3
b) 25 cm3
c) 50 m3
d) 50000 cm3

Tip – always use standard form for very large and very small numbers!
What is a mole?
Atoms and molecules are very small – far too small to count individually!

It is important to know how much of something we have, but we count particles in MOLES
because you get simpler numbers

1 mole = 6.02 x 1023 particles


(6.02 x 1023 is known as Avogadro’s number)

a) If you have 2.5 x 1021 atoms of magnesium, how many moles do you have?

b) If you have 0.25 moles of carbon dioxide, how many molecules do you have?
How can you work out how many moles you have?

a) From a measurement of MASS:


You can find the number of moles of a substance if you are given its mass and you know its
molar mass:

number of moles = mass/molar mass

n = m/mr

Mass MUST be measured in grams!

Molar mass has units of gmol-1

1. Calculate the number of 2. Calculate the mass of: 3. Calculate the molar mass of
moles present in: the following substances:

a) 2.3 g of Na a) 0.05 moles of Cl2 a) 0.015 moles, 0.42 g

b) 2.5 g of O2 b) 0.125 moles of KBr b) 0.0125 moles, 0.50 g

c) 240 kg of CO2 c) 0.075 moles of Ca(OH)2 c) 0.55 moles, 88 g

d) 12.5 g of Al(OH)3 d) 250 moles of Fe2O3 d) 2.25 moles, 63 g

e) 5.2 g of PbO2 e) 0.02 moles of Al2(SO4)3 e) 0.00125 moles, 0.312 g


b) From a measurement of AQUEOUS VOLUME:
You can find the number of moles of a substance dissolved in water (aqueous) if you are
given the volume of solution and you know its molar concentration:

number of moles = aqueous volume x molar concentration

n = V x C

Aqueous volume MUST be measured in dm3!

concentration has units of moldm-3

If you know the molar mass of the substance, you can convert the molar concentration into a mass
concentration:

Molar concentration (moldm-3) x mr = mass concentration (gdm-3)

1. Calculate the number of 2. Calculate the molar 3. Calculate the molar


moles of substance present in concentration and the mass concentration and the mass
each of the following concentration of the following concentration of the following
solutions: solutions: solutions:

a) 25 cm3 of 0.1 moldm-3 HCl a) 0.05 moles of HCl in 20 cm3 a) 35 g of NaCl in 100 cm3

b) 40 cm3 of 0.2 moldm-3 HNO3 b) 0.01 moles of NaOH in 25 b) 20 g of CuSO4 in 200 cm3
cm3

c) 10 cm3 of 1.5 moldm-3 NaCl c) 0.002 moles of H2SO4 in 16.5 c) 5 g of HCl in 50 cm3
cm3

d) 5 cm3 of 0.5 moldm-3 AgNO3 d) 0.02 moles of CuSO4 in 200 d) 8 g of NaOH in 250 cm3
cm3

e) 50 cm3 of 0.1 moldm-3 e) 0.1 moles of NH3 in 50 cm3 e) 2.5 g of NH3 in 50 cm3
H2SO4
c) From a measurement of GASEOUS VOLUME:
You can find the number of moles of a gas if you are given the volume of the gas and its
pressure (in kPa) and absolute temperature (in K):

number of moles = pressure x volume = PV/RT

R x temperature

Volume of gas must be in m3

Pressure must be in Pa

Temperature must be in K

R is the molar gas constant (8.31 Jmol-1K-1)

1. Calculate the number of 2. Calculate the volume of gas 3. Calculate the mass of the
moles present in: occupied by: following gas samples:

a) 48 dm3 of O2 at 298 K and a) 0.05 moles of Cl2 at 298 K a) 48 dm3 of O2 at 298 K and
100 kPa and 100 kPa 100 kPa

b) 1.2 dm3 of CO2 at 298 K and b) 0.25 moles of CO2 at 298 K b) 1.2 dm3 of CO2 at 298 K and
100 kPa and 100 kPa 100 kPa

c) 200 cm3 of N2 at 273 K and c) 28 g of N2 at 273 K and 250 c) 200 cm3 of N2 at 273 K and
250 kPa kPa 250 kPa

d) 100 dm3 of Cl2 at 30 oC at d) 3.2 g of O2 at 30 oC at 100 d) 100 dm3 of Cl2 at 30 oC at


100 kPa kPa 100 kPa

e) 60 cm3 of NO2 at 25 oC and e) 20 g of NO2 at 25 oC and 100 e) 60 cm3 of NO2 at 25 oC and


100 kPa kPa 100 kPa

TRANSITION COURSE – END OF PART 1!

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