As Transitions Measuring Amount of Substance Enthalpy Changes

Topics: Mole, Avogadro constant, Amount of substance Pages: 6 (871 words) Published: January 14, 2013







OCR Chemistry A

Unit 1 – Autumn Term 2012

• Atoms and Electron Structure

• Moles, Equations and Acids

• Structure and Bonding

• Redox, Group 2 and Group 7

Unit 1 Exam – January 2013 – 1 hour – 90 UMS – 30% of total

Unit 2 – Spring Term 2013

• Basic Concepts in Organic Chemistry

• Alkanes and Alkenes

• Haloalkanes and Alcohols

• Enthalpy Changes

• Rates and Equilibrium

• Modern Analytical Techniques

• Chemistry of the Air

• Sustainability

Unit 2 Exam – May 2013 – 1 hr 45 mins – 150 UMS – 50% of total

Unit 3 – Autumn Term 2012 and Spring Term 2013

Internal Assessments – 60 UMS – 20% of total



Convert the following into grams:

a) 0.25 kg

b) 15 kg

c) 100 tonnes

d) 2 tonnes


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



Mass MUST be measured in grams!

Molar mass has units of gmol-1

|1. Calculate the number of moles present |2. Calculate the mass of: |3. Calculate the molar mass of the | |in: | |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 xmolar concentration



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) xmr=mass concentration (gdm-3)

|1. Calculate the number of moles of |2. Calculate the molar concentration and |3. Calculate the molar concentration and | |substance present in each of the following |the mass concentration of the following |the mass concentration of the following | |solutions: |solutions: |solutions:...
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