The mole notes and questions PDF

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THE MOLE In the chemical industry, scientists need to calculate the amount of materials necessary to make new substances. Similarly, in cooking a chef needs to work from a recipe of ingredients. However, in a chemical reaction it is the number of particles that is most important, rather than the mass. Imagine that a blue marble reacts with a white marble to form a molecule. If we have 10g of blue marbles, does that mean that we also need 10g of white marbles for the reaction? Imagine that a blue marble weighs 1g and a white marble weighs 3g and these react to produce a pale blue marble of 4g. Then if we have 10 blue marbles which weigh 10g, we don’t need 10g of white marbles but 30 g (10 x 3g)of white marbles to react with them. The mole concept allows us to calculate the mass of equal numbers of particles by taking into consideration relative atomic/molecular mass. ··· ···· ···

OOO OOOO OOO

·O ·O · O ·O ·O ·O ·O ·O ·O ·O

10 blue marbles weigh 10g

10 white marbles weigh 30g

10 molecules weigh 40g

When dealing with chemical equations we are concerned with the numbers of particles reacting. As atoms are so tiny we deal with a large number of particles called a mole WHAT IS A MOLE? 1 mole = 6 ´ 1023 (AVOGADRO'S CONSTANT) particles MOLES(n) =

MASS (m) MOLAR MASS (Mr)

1 mole of carbon atoms = 12g = 6 ´ 1023 carbon atoms ¨One mole of any substance contains 6 ´ 1023 particles of that substance. ¨One mole of an element is it's relative atomic mass in grams

¨One mole of a compound is its relative molecular mass in grams. DEFINITION One mole is the amount of a substance which contains as many particles as there are carbon atoms in 12g of carbon 12. REMEMBER M n

Mr

m = mass in grams n = number of moles Mr = molecular mass

EXAMPLE 1 (a) Calculate the number of moles of sodium in 1.6g of sodium n = m/Mr n = 1.6/23 = 0.070moles (b) Calculate the number of moles of sodium in 1.6g of sodium hydroxide n = m/Mr n = 1.6/40 = 0.040moles (c ) Calculate the number of moles of oxygen atoms in 6.4g of O2 n = m/Mr n = 6.4/16 = 0.40moles (d) Calculate the number of moles of oxygen molecules in 6.4g of O2 n = m/Mr n = 1.6/32 = 0.020moles QUESTION 1 Calculate the moles present in the following (a) 5g of copper (e) 16g of hydrogen gas (b) 10 g of zinc (f) 1g of chlorine gas (c) 15g of potassium (g) 6g of nitrogen atoms (d) 12g of oxygen gas (h) 12 g of oxygen atoms

QUESTION 2 Calculate the number of moles present in the following – remember to make sure you have the correct formula before calculation (a) 4.4g of carbon dioxide (e) 19.6 g of sulphuric acid (b) 267g of aluminium chloride (f) 100g of magnesium nitrate (c) 15g of iron (II) sulphate (g) 25g of sodium oxide (d) 72g of iron (II) sulphide (h) 120g of potassium carbonate

EXAMPLE 2 (a) Calculate the mass of carbon dioxide in 0.05moles of CO2 m = n x Mr m = 0.05 x 44 = 2.2g (b)

Calculate the mass of carbon in 0.05moles of CO2 m = n x Mr m = 0.05 x 12 = 0.6g

(c)

Calculate the mass of hydrated copper (II) sulphate crystals in 0.01 moles of CuSO4.5H2O. m = n x Mr m = 0.01 x 249.5 = 2.495g

(d)

Calculate the mass of water in 0.01 moles of CuSO4.5H2O. m = n x Mr m = 5(0.01) x 18 = 0.9g

QUESTION 3 What mass of solid is required to obtain the following? (a) 0.06 moles of CaCO3 (b) 1.2 moles of PCl5 (c) 0.01 moles of MgSO4 (d) 0.66 moles of LiAlH4 QUESTION 4 (a) (b) (c) (d) (e) (f) (g) (h)

What mass of solid is required to obtain the following? 3moles of aluminium sulphate 1.5 moles of lithium oxide 12 moles of phosphorus pentachloride 1.3 moles of copper sulphate . 5 H2O 0.02 moles of iron (III) sulphide 0.025 moles of calcium nitride 35 moles of aluminium carbonate 12.5 moles of ammonium sulphate

You can also calculate molecular mass when the mass and number of moles are known EXAMPLE 3 (a) If 2 moles of a substance has a mass of 200g, what is its Mr? Mr = m/n Mr = 200/2 = 100

(b)

If 0.05 moles of a substance has a mass of 13g, what is its Mr? Mr = m/n Mr = 13/0.05 = 260

EXAMPLE 4 (a) How many molecules exist in 0.1 moles of oxygen? No of particles in 1 mole = 6 x 1023 Therefore in 0.1 moles we have 6 x 1023 x 0.1 = 6 x 1022 molecules QUESTION 5 Calculate the relative mass for the following substances: (a) 0.025 moles in1g (b) 0.5 moles in 1g (c) 2.5 moles in100g (d) 2.5 moles in 1250g QUESTION 6 Calculate the number of molecules present in the following: (a) 0.5 moles of water (b) 12 moles of water (c) 12 moles of ammonia gas (d) 0.08 moles of ethanoic acid...