amount of substance

Question 1

What is the formula that links moles and Mr?

Answer 1

Moles = mass(g) / Mr?

Question 2

What is the equation to work out the number of particles?

Answer 2

Number of particles = (mass(g) / Mr) x 6.022x1023

Question 3

What is the ‘theoretical yield’ of a reaction? What is the formula for percentage yield?

Answer 3

Theoretical yield = maximum mass of product you could make in a given reaction.

Percentage yield = (actual yield/theoretical yield)x100

Question 4

What are excess and limiting reagents? How can use deduce which reactant is excess and limiting?

Answer 4

Limiting reagents = less than enough, this will limit that amount of product formed. Excess reagent = more than enough. You need to calculate the moles of each reagent and divide by any big numbers (if applicable).

Question 5

What is meant by atom economy? What is the formula for atom economy? What are the advantages of having high atom economy?

Answer 5

Atom economy = (mass of desired product/total mass of reactants)x100. Use the Mrs and multiple by big numbers to calculate the masses. Higher atom economy means there will be fewer waste products.

Question 6

What are the definitions of ‘empirical formula’ and ‘molecular formula’?

Answer 6

Empirical formula = simplest whole number ratio of atoms in a compound. Molecular formula = actual number of atoms in a compound

Question 7

What is the difference between a hydrated salt and an anhydrate? How do you perform ‘water of crystallisation’ experiments?

Answer 7

Hydrated salt = ionic substance with a certain number of water molecules associated with it. Anhydrous/anhydrated is just the salt (water has been removed). To work out ‘water of crystallisation’ find the mass of the hydrated salt. Then heat until the mass stops decreasing, this should mean that all of the water has evaporated. Find the mass of the anhydrate. The difference in mass of hydrated and anhydrate = mass of water. Find moles of salt and water. The simplest whole number ratio will equal number of water molecules.

Question 8

What is the ideal gas equation? What are the units? What are the conversions for different units?

Answer 8

Moles = concentration (mol/dm3) x volume (dm3)

Moles = (concentration (mol/dm3) x volume (cm3))/1000

Question 9

How many cm3 in 1dm3?

Answer 9

1000cm3 = 1dm3

Question 10

What are the two different units of concentration? How can you convert between them?

Answer 10

moldm-3 = gdm-3 ÷ Mr

gdm-3 = moldm-3 x Mr

Question 11

What is a standard solution? What is a method for making a standard solution?

Answer 11

A standard solution is a a solution of accurately known concentration prepared from a primary standard (a compound which is stable, of high purity, highly soluble in water and of a high molar mass to allow for accurate weighing) that is weighed accurately and made up to a fixed volume.

(a) The mass of solute needed is calculated and weighed.
(b) The solute is dissolved in some distilled water in a beaker.
(c) The solution is transferred into a volumetric flask.
(d) More distilled water is added to obtain the required volume. The flask is stoppered and shaken

Question 12

What are 4 common hazard symbols? How do you reduce risk for each symbol?

Answer 12

wear nitrile gloves
don’t use near open flames

Question 13

What piece of equipment allows you to accurately measure 25cm3 of acid/alkali for a titration?

Answer 13

Pipette

Question 14

What piece of equipment allows you to add liquid drop by drop to a flask during a titration?

Answer 14

Burette

Question 15

Name some indicators that are suitable for a titration

Answer 15

Phenolphthalein, methyl orange, litmus

Question 16

What is the end point of a titration?

Answer 16

When the indicator just changes colour

Question 17

What are concordant results?

Answer 17

Titres that are within 0.10cm3 of each other

Question 18

What is uncertainty? How is percentage uncertainty calculated? How can percentage uncertainty be reduced?

Answer 18

ncertainty is usually plus or minus half of the smallest division on a piece of equipment.

% uncertainty = (uncertainty / amount measured) x 100

To reduce % uncertainty - use either larger masses (for solids) or larger volumes or concentrations (for solutions) will reduce % uncertainty.

Question 19

What are the equations for ionic equations for reactions of acids? How do you write an ionic equation for a precipitation reaction?

Answer 19

acid + hydroxide= H + (aq) + OH – (aq) → H 2 O (l)
acid + carbonate=
acid + hydrogencarbonate
acid + ammonia