State what is meant by the term empirical formula.

A compound of gallium (atomic number 31) has the following composition by mass:

S  22.5%

Ga 32.6%

O  44.9%

Determine the empirical formula of the compound.

A chemist added 1.25 g of hydrated sodium carbonate (Na₂CO₃·xH₂O) to 175 cm³ of 0.400 mol dm^–3 hydrochloric acid in a beaker and stirred the mixture.

After the reaction was complete, the resulting solution was transferred to a volumetric flask, made up to 250 cm³ with deionised water and mixed thoroughly.

Several 25.0 cm³ portions of the resulting solution were titrated with 0.250 mol dm^–3 aqueous sodium hydroxide. The mean titre was 19.95 cm³ of aqueous sodium hydroxide.

Calculate the value of x in Na₂CO₃·xH₂O

Give your answer as an integer. 

Most modern vehicles are fitted with airbags which are inflatable safety cushions designed to protect the occupants of the vehicle during a collision.

Airbags inflate rapidly as a result of the thermal decomposition of sodium azide, NaN₃, to produce sodium and nitrogen gas. 

Write a balanced symbol equation for the decomposition of sodium azide. State symbols are not required. 

A typical airbag contains 90 g of sodium azide.

Calculate the maximum volume of nitrogen gas the airbag could be inflated with, at room temperature and pressure, using this mass of sodium azide. 

2NaN₃ ➔ 2Na + 3N₂

Give your answer in dm³ and to 2 significant figures.

The sodium produced from the decomposition of sodium azide subsequently reacts with potassium nitrate in the second reaction in the airbag. This reaction produces three products; potassium oxide, sodium oxide and nitrogen gas.

Construct a balanced symbol equation for this reaction. State symbols are not required.

Calculate the number of sodium ions in 1.80 g of sodium oxide, Na₂O.

Give your answer in standard form and to three significant figures.

L = 6.022 x 10²³ mol^‒1

This question is about the different salts of zinc.

A pupil heated a solid sample of ZnSO₄⸱xH₂O for two minutes to remove water and determine a value for X. 

What does ⸱xH₂O represent in the formula ZnSO₄⸱xH₂O?

The table below shows the pupil's results.

Use the data in the table above to calculate a value for x in the formula ZnSO₄⸱xH₂O

Give your answer to two decimal places. 

The correct value for x is 7.

Suggest a reason for the difference between the experimental value for x and the correct value.

Zinc chloride can be prepared in the laboratory by the reaction between zinc and hydrogen chloride gas.

Zn_(s) + 2HCl_(aq) ➔  ZnCl_2(aq) + H_2(g)

Construct an ionic equation for this reaction.

An impure sample of zinc powder, with a mass of 6.21 g, was reacted completely with hydrogen chloride gas.

The equation for the reaction is:

Zn_(s) + 2HCl_(aq) ➔  ZnCl_2(aq) + H_2(g)

The zinc chloride produced had a mass of 11.8 g.

Calculate the percentage purity of the zinc metal. Give your answer to three significant figures. 

Calcium sulfate reacts with calcium sulfide in the following reaction.

3CaSO₄ + CaS → 4CaO + 4SO₂

55.0 g of calcium sulfate is heated with 9.58 g of calcium sulfide until there is no further reaction.

Show that calcium sulfide is the limiting reactant in this reaction.

Calculate the mass, in g, of calcium oxide formed. 

3CaSO₄ + CaS → 4CaO + 4SO₂

Use the same quantities of reactants as mentioned in part a). Calcium sulfide in the limiting reactant.

This question is about aluminium and its compounds.

When aluminium metal and dilute hydrochloric acid react together, an aqueous solution of aluminium chloride is produced.

Construct a balanced symbol equation for this reaction. Include state symbols.

A student completely reacts 1.15 g of aluminium with 0.500 mol dm^-3 HCl.

Calculate the minimum volume, in cm³, of 0.500 moldm^-3 HCl that the student needs for the reaction.

Give your answer to three significant figures.

Calculate the volume of H₂, in dm³, that would be produced at room temperature and pressure (RTP).

Calculate the mass, in g, of AlCl₃ formed. 

Give your answer to three significant figures.

The student repeats the experiment using the same volume of 1.00 mol dm^-3 HNO₃ as in (b).

State and explain whether the volume of H₂ produced would be greater than, smaller than, or the same as, the value calculated in (c). 

Calculate the number of ions present in 0.0426 moles of AlCl₃. 

Give your answer in standard form to three significant figures.

L = 6.022 x 10²³ mol^‒1

A chemist does an experiment to determine the percentage of copper in an alloy.

The chemist;

• reacts 874 mg of the alloy with concentrated nitric acid acid to form a solution (all of the copper in the alloy reacts to form aqueous Cu²⁺ ions)
• pours the solution into a volumetric flask and makes the volume up to 250 cm³ with distilled water
• shakes the flask thoroughly
• transfers 25.0 cm³ of the solution into a conical flask and adds an excess of potassium iodide
• uses exactly 8.00 cm³ of 0.0950 mol dm^-3 sodium thiosulfate (Na₂S₂O₃) solution to react with all the iodine produced.

The equations for the reactions are

2Cu²⁺ + 4I^- ➔ 2CuI + I₂

2S₂O₃^2- + I₂ ➔ 2I^- + S₄O₆^2-

Calculate the percentage of copper by mass in the alloy.

Give your answer to 3 significant figures.

Ethanoic acid is produced by the oxidation of ethanol with acidified potassium dichromate(VI).

An ionic equation for this reaction is

3CH₃CH₂OH + 2Cr₂O₇^2- + 16H⁺ ➔ 3CH₃COOH + 4Cr³⁺ + 11H₂O

Calculate the minimum volume of 0.600 mol dm^-3 potassium dichromate(VI) solution needed to oxidise 8.00 cm³ of ethanol to ethanoic acid.

Give your answer in dm³ to 3 significant figures.

Density of ethanol = 0.789 g cm^-3

Calculate the maximum volume, in cm³, of ethanoic acid that can produced from 8.00 cm³ of ethanol.

Give your answer to 3 significant figures.

Density of ethanoic acid = 1.05 g cm^-3

5.65 cm³ of ethanoic acid was produced from the oxidation of 8.00 cm³ of ethanol using an excess of potasium dichromate(VI).

Using your answer to part b), calculate the percentage yield of this reaction.

Give your answer to 3 significant figures.

0.260 g of a hydrocarbon Z was combusted completely in oxygen to produce 0.817 g of carbon dioxide and 0.334 g of water. 

Calculate the empirical formula of Z.

Use the empirical formula in (b) and the relative molecular mass of Z (M_r = 98.0) to calculate the molecular formula of Z.

A chemist investigates two experimental methods of making methylpropanal. The equations for these two methods are shown below.

The student uses 1.50 g of organic starting material in each method.

Method A yields 828 mg of methylpropanal.

Method B yields 992 mg of methylpropanal.

Calculate the percentage yield for method A. Give your answer to the nearest integer.

Calculate the percentage yield for method B. Give your answer to the nearest integer.

State the importance of a high percentage yield when choosing the method used to make methylpropanal.

Phosphoric(V) acid (H₃PO₄) is an important industrial chemical used in the manufacture of fertilizers. Phosphoric(V) acid can be made by two methods. The first method is a two-step process.

In the first step of the first method, phosphorus is burned in air to produce gaseous phosphorus(V) oxide.

P_4(s) + 5O_2(g) ➔ P₄O_10(g)

266 g of phosphorus were reacted with an excess of air.

Calculate the volume, in dm³, of gaseous phosphorus(V) oxide produced at room temperature and pressure.     

Give your answer to three significant figures. 

In the second step of the first method, phosphorus(V) oxide reacts with water to form phosphoric(V) acid.

P₄O_10(s) + 6H₂O_(l)  ➔  4H₃PO_{4 (aq)}

Calculate the mass, in kg, of phosphorus(V) oxide required to produce 2.50 m³ of 4.00 mol dm^–3 phosphoric(V) acid solution.

Give your answer to three significant figures.

In the second method to produce phosphoric(V) acid, 4.60 kg of Ca₃(PO₄)₂ are added to an excess of aqueous sulfuric acid.

Ca₃(PO₄)_2(s) + 3H₂SO_4(aq)  ➔  2H₃PO_{4 (aq)} + 3CaSO_4(s)

2.11 kg of phosphoric(V) acid are produced.

Calculate the percentage yield of phosphoric(V) acid. Give your answer to three significant figures.