2.1 - Formulae and Amounts Of A Substance

Question 1

Empirical Formula Definition

Answer 1

A formula which shows the simplest whole number ratio of atoms of each element in a compound.

Question 2

Molecular Formula Definition

Answer 2

A formula which shows the actual number of atoms of each element in a molecule.

Question 3

Give the formula for Percentage Yield

Answer 3

Actual yield/ theoretical yield x100

Question 4

Give the formula for Atom Economy

Answer 4

Mass of desired product/ total mass of products x100

Question 5

Definition of Percentage Yield

Answer 5

The percentage yield of a chemical reaction shows how much product was actually made compared with the amount of product that was expected.

[Give the formula also]

Formula = Actual yield/ theoretical yield x100

Question 6

Definition of Theoretical Yield

Answer 6

The theoretical yield is the maximum mass of product expected from the reaction, using reacting masses.

Question 7

Definition of Actual Yield

Answer 7

The actual yield is the mass of the product that is actually obtained from the real chemical reaction.

Question 8

Definition of Atom Economy

Answer 8

Atom Economy is the overall efficiency of a chemical process.

[Give the formula also]

Atom Economy = Mass of desired product/ total mass of products x100

Question 9

Workbook 2.1 Exercise 1 page 2 Q1
Q1 The empirical formula and relative molecular mass of some compounds are shown below. Work out the molecular formula of each one.
a) NH2, Mr = 32
f) CH2, Mr = 42

Answer 9

Q1 a) NH2 = 14 + 2(1) = 16
Molecular Formula = N2H4

f) CH2 = 12 + 2(1) = 14
Molecular Formula = C3H6

Question 10

Workbook 2.1 Exercise 1 Q2
Q2 Find the empirical formulae of the following compounds using the data given.
a) Ca: 20% ,Br: 80%
b) Na: 29.1% ,S: 40.5%, O: 30.4%
e) N: 15.2g, O: 34.8g

Answer 10

Q2 a) CaBr2

b) Na2S2O3
e) NO2

Question 11

Workbook 2.1 Exercise 1 Q3

Q3 3.53g of iron reacts with chlorine to form 10.24g of iron chloride. Find the empirical formula of the iron chloride.

Answer 11

Q3 FeCl3

Question 12

Workbook 2.1 Exercise 1 Q4
Q4 50.0g of a compound contains 22.4g of potassium, 9.2g of sulfur, and the rest oxygen. Calculate the empirical formula of the compound.

Answer 12

Q4 K2SO4

Question 13

Workbook 2.1 Exercise 1 Q5
Q5 An oxide of phosphorus contains 56.4% phosphorus and 43.6% oxygen. It’s relative molecular mass is 220. Find both the empirical and molecular formula of the oxide.

Answer 13

Q5 P4O6

Question 14

Workbook 2.1 Exercise 1 Q6
Q6 A compound contains 40.0g of carbon, 6.7g of hydrogen and 53.5g of oxygen. It has a relative molecular formula of 60. Find both the empirical and the molecular formula of the compound.

Answer 14

Q6 C2H4O2

Question 15

Workbook 2.1 Exercise 1 Q7
Q7 Compounds were analysed for the percentage composition of each of their elements and were recorded for all elements except oxygen. From each of the following sets of results, write down the ratios of all the elements in the compound. Work out the empirical formula, and identify the compound where possible.

a) 40% calcium, 12% carbon
b) 34% copper, 15% nitrogen
c) 24.7% potassium, 34.8% manganese
d) 40% carbon, 6.7% hydrogen

Answer 15

Q7 a) CaCO3

b) Cu(NO3)2
c) KMnO4
d) C2H4O2 or CH3COOH

Question 16

Workbook 2.1 Exercise 1 Q8
Q8 When 1.16g of silver oxide was completely reduced to silver metal, 1.08g of the metal was produced. Use these data to calculate the empirical formula of this oxide.

Answer 16

Q8 Ag2O

Question 17

Workbook 2.1 Exercise 1 Q9
Q9 An oxide of copper was reduced to the metal by hydrogen. The following masses were recorded:
mass of oxide = 31.8g
mass of copper = 25.4g
Calculate the empirical formula of the oxide.

Answer 17

Q9 CuO

Question 18

Workbook 2.1 Exercise 1 Q10
Q10 3.75g of hydrated copper(II) sulfate was gently heated until all the water of crystallisation had been driven off. The residue weighed 2.40g. Calculate the number of moles of water of crystallisation present in 1 mole of hydrated copper (II) sulfate.

Answer 18

CuSO4.5H2O

Question 19

Define Molar Gas Volume

Answer 19

The volume of one mole of any gas under specified conditions of temperature and pressure, e.g. 24dm^3 at 20°C (293°K) and one atmosphere pressure (10^5 Pa)

Question 20

One mole of any gas at 293°K and a pressure of 1 atmosphere (10*5 Pa) occupies a volume of …

Answer 20

24 dm*3

Question 21

Workbook 2.1 Atom Economy Exercise 5 Q4
What is the percentage atom economy of the reaction to produce copper from copper oxide?

2CuO + C —> 2Cu + CO2

Answer 21

Mass of desired product = 2x64 = 128
Total mass of products = Mass 2Cu + Mass CO2 = 2x64 + 12 + (2x16) = 128 + 44 = 172

Atom Economy = 128/172 x100 = 74.4%

Question 22

Formulae and equations handout 4
Problem 12
How many water molecules, H2O, are there in 1 drop of water?
Assume one drop of water is 0.05 cm^3, and that the density of water is 1 gcm^-3. (H = 1; O = 16)

Answer 22

Mass of water = conc. x volume = 1 x 0.05 = 0.05 grams
Moles of water = mass/RFM = 0.05/18 = 1/360
Molecules of water = moles x Avagrado’s constant = 1/360 x 6.02 x 10^23 = 1.67 x 10^21

Question 23

Formulae and equations 4
Problem 13
Sea water contains about 30g of ionic sodium chloride, Na+Cl-, in every 1000 cm^3 of water. What volume of sea water contains 10^20 ion pairs, Na+Cl-? (Na = 23; Cl = 35.5)

Answer 23

Molecules of NaCl (ion pairs) = 10^20

Moles = Molecules of NaCl/ Avogadro’s constant = 10^20/ 6.02 x 10^23 =