Unit 1-Formulae

Question 1

What information does a chemical’s formula tell you?

Answer 1

It gives the number of atoms present in the molecule.

The formula of a covalent network or ionic compound gives the simplest ratio of atoms/ions in the substance.

Question 2

What do the following prefixes mean?

1. mono
2. di
3. tri
4. tetra

Answer 2

1. one atom
2. two atoms
3. three atoms
4. four atoms

Question 3

What is the formula for:

Sulphur Monoxide

Carbon Dichloride

Copper Tribromide

Phosphorus Tetrahydride

Answer 3

SO

CCl₂

CuBr₃

PH₄

Question 4

What is meant by the valency of a chemical?

Answer 4

Valency is the combining power of an element. Elements in the same group of the periodic table have the same valency. The valency of an element is related to how many electrons are in the outer shell.

Question 5

What are the valencies of elements in groups 1 to 8?

Answer 5

Question 6

If given the name of a compound, how can you work out the formula?

Answer 6

S.V.S.D.F system. (Valency cross over)

Step one - write down the symbols

Step two - write valency (USE DATA BOOKLET)

Step three - swap the valencies

Step four - if the valencies can be simplified, divide them both by the smaller of the two numbers. If one of the numbers is already one, then they cannot be divided and simplified any further.

Step five - write the formula.

Question 7

What is the formula of strontium chloride?

Answer 7

SrCl₂

Question 8

What is the formula of Germanium sulfide?

Answer 8

GeS₂

Question 9

How can you find out the valency of group ions?

Answer 9

In the data booklet (page 8)

We can take the number of charges on the ion as the valency of the ion.

OH^- has a valency of 1

SO₄^2- has a valency of 2

Question 10

What is the formula of Magnesium Carbonate?

Answer 10

MgCO3

Question 11

Work out the formula of Beryllium Phosphate.

Answer 11

Be₃(PO₄)2

Question 12

Why do some compounds have roman numerals in their name?

Copper (II) chloride

Answer 12

Some elements do not always make the same number of bonds. These include some transition metals.

The valency of these elements is usually given in roman numerals inside brackets.

Question 13

What is the formula of:

1. Copper (II) chloride?
2. Copper (I) chloride?

Answer 13

1. CuCl₂
2. CuCl

Question 14

What must be included when asked to write an ionic formula?

Answer 14

Charges on each ion.

One part will be positive and the other negative.

Question 15

What is the ionic formula for potassium nitrate?

Answer 15

Potassium makes 1 bond and will be positive.

Nitrate makes 1 bond and has a negative charge (from data booklet)

Formula = KNO₃

Ionic formula = K⁺NO₃^-

Question 16

What is the ionic formula for iron(III) oxide?

Answer 16

Iron makes 3 bonds and will be positive

Oxygen makes 2 bonds and will be negative

Formula = Fe₂O₃

Ionic formula = (Fe³⁺)₂(O^2-)₃