Topic 1 - Formulae. equations and amounts of substance

Question 1

What is the Avogadro’s Constant?

Answer 1

The number of particles in one mole of any substance

(6.02 x 10^23)

Question 2

Relative atomic mass (Ar)

Answer 2

The mean mass of an element, divided by one twelfth of the mass of an atom of the carbon -12 isotope

Question 3

Relative molecular mass (Mr)

Answer 3

The mean mass of a molecule of a compound, divided by one twelfth of the mass of the carbon-12 isotope

Question 4

Empirical formula

Answer 4

The simplest whole number ratio of atoms of each element in a compound. It is found using molar ratios of each element

Question 5

Molecular formula

Answer 5

The actual number of each atom in the molecule. It can be determined using the Mr of the empirical formula and the true Mr of the molecule.

True Mr = Mr of empirical formula x multiplier

Question 6

Molar Mass

Answer 6

The molar mass of a substance is its mass in grams per mole and has the units g mol^-1.

Molar mass = Mass/Number of moles

Question 7

Parts per million (PPM)

Answer 7

Concentration can be given in ppm. This gives the units of mass of that particular species within 1,000,000 total units of mass

It is most commonly used to represent the concentration of gases.

Question 8

Molar volume of gases

Answer 8

One mole of any gas at room temperature and pressure will take up the same volume, regardless of its composition.

This volume is 24,000 cm^3, or 24 dm^3

Volume of gas (dm^3) = 24 x Number of moles

Question 9

Ideal gas law

Answer 9

Pressure is proportional to temperature
Volume is proportional to temperature
Pressure and Volume are inversely proportional

pV = nRT

p= pressure in pascals
V= volume in m^3
T= temperature in Kelvin
n= moles

R is the ideal gas constant, equal to 8.31 JK^-1mol^-1

Question 10

Ionic equations

Answer 10

They show just the reacting particles that undergo change during the reaction and not the spectator species.

Question 11

Percentage yield

Answer 11

It indicates how much of the maximum amount of product you obtained during an experiment.

A low percentage yield could indicate an incomplete reaction, or the loss of product during purification.

% yield = (actual mass/theoretical mass) x 100

Question 12

Atom economy

Answer 12

A measure of efficiency since it measures the proportion of reactant atoms which are converted into the desired product.

% atom economy = (Mr of desired product/Total Mr of all products) x 100

Question 13

Ionic Half equations

Answer 13

Ionic half equations are used for reactions involving oxidation and reduction, and usually only show what happens to only one reactant.

Question 14

Acid with metals

Answer 14

metal + acid –> salt + hydrogen

Bubbles of hydrogen gas form, and if the salt formed if soluble, a solution is formed

The metal must be sufficiently reactive to react in this way.

The H^+ ions are removed from the solution when they react with the metal. The H^+ ions gain electrons from the metal and are converted to H_2(g), the H^+ ions are reduced.

Question 15

Acids with metal oxides and insoluble metal hydroxides

Answer 15

metal oxide + acid –> salt + water
metal hydroxide + acid –> salt+water

The reactivity of the metal does not matter in these reactions because the reactant is present as metal ions.
The only observation is likely to be the formation of a solution

These reaction are classed as neutralisation reactions because the H^+ ions react with the O^2- or OH^- ions.
They are not redox reactions since there is no change in the oxidation number of any of the species.

Question 16

Acids with alkalis

Answer 16

Metal hydroxides that dissolve in water are called alkalis.

alkali + acid –> salt + water

There are no visible changes during these reaction, although if a thermometer is used, a temperature rise can be detected.

These reaction are classed as neutralisation reactions because the H^+ ions react with the OH^- ions.
They are not redox reactions since there is no change in the oxidation number of any of the species.

Question 17

Acids with carbonates

Answer 17

metal carbonate + acid –> salt+water+carbon dioxide

Bubbles of carbon dioxide gas forms. If the salt formed is soluble, a solution forms

These can be classed as neutralisation reactions because the H^+ ions react with CO^2/3- ions.

They are not redox reactions since there is no change in the oxidation number of any of the species.

Question 18

Acids with Hydrogen-carbonates

Answer 18

Hydrogen-carbonates are compounds containing the hydrogen-carbonate ions (HCO^-_3), and they react with acids in the same way as carbonates

A suitable test for the presence of carbonate of hydrogen-carbonate ions in a solid or solution is to add an aqueous acid and to test the gas produced with limewater