unit 1 Chemical calculations topic 5&6

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

12343210

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

Question 17

What is meant by the gram formula mass (GFM) of a substance?

Answer 17

The GFM is the mass of one mole of a substance

Question 18

How do you calculate the Gram Formula Mass (GFM) of a compound?

Answer 18

Work out the formula then add all the mass numbers together.

Question 19

What is the GFM of Carbon Dioxide?

Answer 19

CO₂

12 + (2x16) = 44

Question 20

To calculate the mass of a number of moles, or calculate the number of moles in a certain mass, a formula triangle can be used.

Describe it.

Answer 20

n=m/gfm

Question 21

How many moles are present in 25 g of calcium carbonate? (GFM=100)

Answer 21

number of moles = mass/GFM

25/100 = 0.25 moles

Question 22

Calculate the mass of 0.25 moles of butane (C4H10)

RAM of Carbon = 12

RAM of Hydrogen = 1

Answer 22

mass = number of moles x GFM

mass = 0.25 x 58

mass = 14.5g

Question 23

Equations can be described as being balanced. What does this mean?

Answer 23

A chemical equation is said to be balanced when there are the same number of the same type of every atom on both sides of the equation.

Question 24

How can you balance an equation?

Answer 24

Balancing is adding BIG numbers. You cannot change any of the small numbers in a chemical formula. If balancing is required, put the number in front of the substance.

Question 25

How would this equation be balanced?

Cu + O2→ CuO

Answer 25

2Cu + O₂ →​ 2CuO

Question 26

Balance this equation

C₄H₈ + O₂ → CO₂ + H₂O

Answer 26

Balance Carbons

C₄H₈ + O₂ → 4CO₂ + H₂O

Balance Hydrogens

C₄H₈ + O₂ → 4CO₂ + 4H₂O

Balance Oxygens

C₄H₈ + 6O₂ → 4CO₂ + 4H₂O