Topic 5 Formulae, Equations and Moles

Question 1

What does the Avogadro constant represent?

Answer 1

The number of atoms per mole of the carbon-12 isotope
The number of atoms or molecules that are in a mole of a substance

Question 2

What is an empirical formula?

Answer 2

Simplest whole number ratio of atoms of
each element present in a compound

Question 3

How to calculate empirical formula?

Answer 3

● Divide the amount of each element by its molar mass
● Divide the answers by the smallest value obtained
● If there is a decimal, divide by a suitable number to make it into a whole number

Question 4

State Avogadro’s law

Answer 4

Under the same temperature and pressure, one mole of any gas would occupy the same volume.

Question 5

What are the ideal ways in which gases behave?

Answer 5

● They are in continuous motion
● No intermolecular forces experienced
● Exert pressure when they collide with each other or container
● No kinetic energy is lost in the collisions
● When temperature increases, kinetic energy of gases also increase

Question 6

Write down the ideal gas equation (in words and symbols, including the units?

Answer 6

pV = nRT
Pressure (Pa) x volume (m 3 ) = number of moles (mol) x gas constant (8.314 J mol -1 ) x temperature (K)

Question 7

1 atmospheric pressure is equal to how many pascal?

Answer 7

1 atm = 101325 Pa

Question 8

0°C is equal to how many kelvin?

Answer 8

273 K

Question 9

What is a standard solution?

Answer 9

A solution of known concentration

Question 10

Write down the steps to prepare a standard solution

Answer 10

1. Weigh the solute using the weigh by difference method
2. In a beaker dissolve the solute using the solvent
3. Pour the solution into a volumetric flask
4. Rinse the beaker using the solution and add it to the flask
5. Add solvent to the flask carefully until it reaches the graduation line
6. Mix the solution thoroughly to ensure complete mixing

Question 11

What are the four common state symbols?

Answer 11

1. Solid (s)
2. Liquid (l)
3. Gaseous (g)
4. Aqueous (aq)

Question 12

What are the reasons for not obtaining 100% yield? (5)

Answer 12

● Reaction may be at equilibrium
● The reactants may be impure
● Side reactions could happen
● Reactants or products may be left behind while transferring
● Loss of products during separation and purification

Question 13

What does atom economy tell us about?

Answer 13

The proportion of desired products compared with all the products formed in the reaction

Question 14

Does 100% yield mean 100% atom economy?

Answer 14

No, even if all the reactants are converted into products, not all products of the reaction will be the required products

Question 15

Which type of reaction has 100% atom economy?

Answer 15

Addition reactions (two or more reactants are combined to form a product)

Question 16

What does systematic error mean?

Answer 16

The same error appears in each measurement

Question 17

The error of mass measurement can be reduced by using which
method?

Answer 17

Weighing by difference method

Question 18

Explain how to measure using the weighing by difference method

Answer 18

1. Measure the mass of the container
2. Add the chemical to the container and weigh the container
3. Calculate the mass of the chemical
4. Transfer the chemical and remeasure the empty container
5. Calculate the mass of the chemical transferred

Question 19

Is the percentage error higher or lower, when the quantity measured has been reduced?

Answer 19

Higher