Empirical formulae

Atoms are made up of protons, neutrons and electrons.

The table shows the relative mass and relative charge for a proton and electron.

Complete the table to show the relative mass and relative charge of a neutron.

Use numbers from the list.

The mass of a proton is 1.67 × 10^–27 kg. (b)

An electron is 0.0005 times lighter than a proton.

Calculate the mass of an electron in kg.

Give your answer in standard form.

The symbol for an atom of nitrogen is:

The table shows the number of protons in this atom of nitrogen.

Complete the table by writing in the number of neutrons and number of electrons.

Nitrogen reacts with hydrogen to form ammonia, NH₃.

This is the equation for the reaction.

What is the missing number that balances the equation?

Tick (✓) one box.

Calculate the relative formula mass, M_r, of ammonia, NH₃.

Relative atomic masses (A_r): H = 1 N = 14

A student thinks that the empirical formula of ammonia is NH₃.

Explain why the student is correct.

Tick (✓) one box.

Fig. 14.1 shows the ball and stick model for a molecule of ammonia.

Fig. 14.1

Which statements about a ball and stick model are true, and which are false ?

Tick (✓) one box in each row.

Fig. 14.2 shows the structure of a molecule of ammonia.

Fig. 14.2

Complete the dot and cross diagram for a molecule of ammonia.

Only show the outer shell electrons.

Atoms are made from the sub-atomic particles protons, neutrons and electrons.

Table 12.1 shows the relative mass and relative charge of the three sub-atomic particles.

Complete Table 12.1 by writing the name of a sub-atomic particle in each space.

Table 12.1

An atom of phosphorus has an atomic number of 15 and a mass number of 31.

Calculate the number of protons, neutrons and electrons in this atom of phosphorus.

Phosphorus reacts with oxygen to form an oxide called phosphorus trioxide.

Phosphorus trioxide has the molecular formula P₄O₆.

What is the empirical formula of phosphorus trioxide?

Tick (✓) one box.

Complete the balanced symbol equation for the reaction of phosphorus trioxide with water.

Table 12.2 shows the melting point and boiling point of phosphorus trioxide.

Table 12.2

In the box draw the particle arrangement for phosphorus trioxide at 100 °C.

What is the empirical formula of this compound?

B CH₃O

C C₂H₆O₂

D HOCH₂CH₂OH

A molecule of glucose has the molecular formula C₆H₁₂O₆.

What is the empirical formula of glucose?

A CHO

B CH₂O

C C₆H₁₂O₆

D (CO₆)H₁₂

A molecule of glucose has the molecular formula C₆H₁₂O₆.

What is the empirical formula of glucose?

A CHO

B CH₂O

C C₆H₁₂O₆

D (CO)₆H₁₂

Octane, C₈H₁₈, is a compound in petrol.

Which statement about octane is correct?

A It is a hydrocarbon with a relative molecular mass of 66.

B It is a hydrocarbon with the empirical formula C₄H₉.

C It is extracted from crude oil by filtration.

D It is in the bitumen fraction of crude oil.

The molecular formula of a compound is C₁₂H₂₆.

What is its empirical formula ?

A C₂H₄

B C₂H₆

C C₄H₆

D C₆H₁₃

Aspirin is a widely used painkiller and is sold as tablets.

One of the chemicals in aspirin is salicylic acid. The structure of salicylic acid is shown below.

Salicylic acid

What is the empirical formula of salicylic acid?

It is important that aspirin is pure for it to be safely used in tablets.

Explain the meaning of the term pure.

Three students, A, B and C make aspirin and check its purity. The students test the purity of the aspirin by measuring the melting point.

Their results are shown in the table.

The melting point of pure aspirin is in the range 138–140 °C.

Which student made pure aspirin?

Tick (✓) one box.

Another method of checking the purity of a substance is paper chromatography.

Look at the diagram of the apparatus used for paper chromatography.

Label the chromatography diagram.

Choose your labels from the list.

active phase

inactive phase

gas phase

mobile phase

stationary phase

* This is the method the students follow for the paper chromatography experiment.

1. Draw a pen line near the bottom of the paper.

2. Put a spot of the substance to be tested onto the line.

3. Stand the paper in the solvent. The solvent should be at the same level as the spot.

4. Cover the beaker.

5. Remove the paper from the beaker before the solvent reaches the top.

6. Calculate the R_f value of the substance by using this equation:

Their teacher notices some mistakes with this method.

Describe and explain the mistakes in the method and suggest improvements that could be made.

Aluminium, phosphorus and magnesium are all in Period 3 of the Periodic Table.

Aluminium has an atomic number of 13 and mass number of 27.

Describe the nucleus of an aluminium atom in terms of sub-atomic particles.

What is the overall charge on the nucleus of an atom of aluminium?

Which sub-atomic particles surround the nucleus?

Phosphorus has a higher atomic number and a higher mass number than aluminium.

A student says that phosphorus must be an isotope of aluminium because it has a different number of neutrons.

Is the student correct?

Tick (✓) one box.

Explain your answer.

Phosphorus can react with oxygen to form oxides. One oxide of phosphorus is called phosphorus pentoxide.

Phosphorus pentoxide has the molecular formula P₄O₁₀.

What is the empirical formula of phosphorus pentoxide?

Aluminium oxide and phosphorus pentoxide have different types of bonding.

The boiling point of aluminium oxide is 2977 °C.

The boiling point of phosphorus pentoxide is 360 °C.

What conclusion can you make about the type of bonding in each oxide?

Explain your answers.

The student is given a sample of the metal magnesium and a solution of an unknown nitrate of metal M, MNO₃(aq).

The student does an experiment to identify metal M.

The student weighs the magnesium. Then the student adds the magnesium to the solution of the metal nitrate.

A displacement reaction takes place forming metal M:

The student separates the metal M and then weighs metal M.

Look at the student’s results: Mass of magnesium = 0.729 g Mass of metal M = 6.476 g

Calculate the moles of magnesium used in the experiment.

Calculate the relative atomic mass of metal M.

Give your answer to 3 significant figures.

Use your answer from (d)(ii) and the Periodic Table to identify the metal M.