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AS Chemistry Unit1 Ch1 Formulae, eqautions and amounts of substacne

1.1Basic definitionTerms Definition

Mole The amount of any substance containing the same number of identical entities as there

are in exactly 12g of the carbon-12 isotope.

Relative atomic mass The average mass of an atom of an element relative to 1/12 the mass of a carbon-12

atom.

Relative isotopic mass mass ofan isotope of an element relative to 1/12 th the mass of a carbon-12 atom

Avogadro constant The number of atoms in exactly 12g of the carbon-12 isotope, which is equal to

6.021023

mol-1

.

1.2Parts per million (ppm) For very small concentrations, percentages become less useful, eg a 0.01% solution is difficult to visualize. Parts

per million is useful for low concentrations.

concentration =

1 000 000 ppm

1 million grams = 1 106

g = 1000 kg = 1 tonne ()

ppm is often used to represent

(1) the very low concentration of pollutants in air or water(2) concentration of components in blood.

Example

1. Blood plasma typically contains 20 parts permillion (ppm) of magnesium, by mass.

Calculate the mass of magnesium, in grams,

present in (a) 100 g (b) 5kg of plasma.

2. 1 kg of a solution contains 0.100 mol of calcium ions,Ca

2+. What is the concentration of the calcium ions by

mass in parts per million (ppm)? [Ca = 40]

3. In 2006, the concentration of carbon dioxide inthe atmosphere was 382 ppm. Express 382 ppm

in %.

4. The European Union has set a limit (with effect fromJanuary 2010) of 3.13 ppm for the proportion of the toxic

gas carbon monoxide in the air that we breathe. Express

3.13ppm in %.

ppm% : multiply by ( )

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AS Chemistry Unit1 Ch1 Formulae, eqautions and amounts of substacne

Examples with mmole / mg where m = 10-3

1. The concentration of blood glucose of 5.0 mmol dm3is within the normal range. What mass of glucose

dissolved in 1 dm3

of blood would give this normal

reading?

2. For drivers in the UK, the legal limit of theconcentration of ethanol in the blood is 80 mg per

100 cm3. Express the concentration in terms of M.

1.3Percentage yieldFact Reason

Yields approaching 100% are very rare in

practical chemical reactions.

The amount of product obtained (the

yield) is usually less than the theoretical

maximum yield.

The reactants may not be pure.

Some reaction is reversible reaction.

Some products are left behind on the apparatus during transfer.

Volatile products may evaporate.

Some of the main product is lost during purification in organic reaction.

There are side reactions and side product, especially in organic reaction

The theoretical yield is calculated using the balanced chemical equationand the following formula

percentage yield =

100%

The actual yieldis the measured mass of

the product obtained in the experiment.

The theoretical yieldis the mass that calculated according to mole ratio

in the reaction, assuming that all reactant is converted into the product.

Example 1

Ethanol reacts with hydrogen bromide to form bromoethane and water. In a reaction, 2.30g of ethanol reacts to form

4.00g of bromoethane. Find the percentage yield of bromoethane.

Example 2

Ethanol is manufactured by the hydration of ethene. In a typical process 28 tonnes of ethene produces 43.7 tonnes of

ethanol. Find the percentage yield.

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AS Chemistry Unit1 Ch1 Formulae, eqautions and amounts of substacne

1.4Atom economyThe atom economy is the percentage of the atoms in the reactants that is present in the desired product.

% atom economy =

A 100% atom economy means that all atoms in reactants turn into products. There are no unwanted products.For example, methanol and carbon monoxide reacts to form ethanoic acid with 100% atom economy.

CH3OH + CO CH3COOH [ atom economy = 60 / 60 x 100% = 100%]

A low atom economy means that are side reactions and form unwanted products. The atom economy idea is popular because(1) A low atom economy leads to a higher raw material costs(2) a low atom economy means the production of by-products. Disposal of waste products is bad for the environment.Example 1

Oxygen can be prepared using several different reactions. Calculate the atom economy of each reaction. Which Which

of them has the highest atom economy by mass?

(a) NaNO3 NaNO2 + O2 b) H2O2 H2O + O2 (c) Cl2 + H2O 2HCl + O2 (d) PbO2 PbO + O2

Example 1

State, with a reason, the atom economy for the production of a poly(alkene) from an alkene. (1)

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Example 2

Chloroethane, C2H5Cl, can be made from either ethane or ethene.

(a) (i) What reagent and condition would be used to make chloroethane from ethane? (2)

Reagent .............................................................................................................................................................................

Condition ............................................................................................................................................................................

(b) (i) What reagent would be used to make chloroethane from ethene? (1)

............................................................................................................................................................................................

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AS Chemistry Unit1 Ch1 Formulae, eqautions and amounts of substacne

(c) Which method of making chloroethane has (3)

a higher atom economy?

.............................................................................................................................................................................

a higher percentage yield?

.............................................................................................................................................................................

Explain your answers.

Higher atom economy

..............................................................................................................................................................................................

..........................................................................................................................................................................................

............................................................................................................................................................................................

Higher percentage yield

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1.5Percentage error There are always a built-in error when using apparatus such as electronic balance, burette and thermometer. For example, the manufacturer of a balance may state that the mass of an object is accurate to 0.01g. Therefore,

a reading of 1.23g may mean a mass anywhere between 1.22g and 1.24g.

The significance of this error can be represented by percentage error:

percentage error =

100%

Example

A thermometer is labelled as having an accuracy of 0.2oC. In an enthalpy of neutralization reaction, the temperature

before the reaction was 17.4oC and after the reaction was 24.5oC. The mass of liquid used was 100g. the specific heat

capacity of the liquid is 4.18J g-1 o

C-1

.

(a) Calculate the percentage error caused by the thermometer(b) Find the error in the evaluation of the heat produced.