Amounts of Substance

How to find the volume of gas produced from a known mass of a reactant?

use the mass and Mr to find the moles, then calculate the amount of moles required for the reaction, use the pressure equation 

How to make a standard solution?

• Record the mass of a weighing boat and solid

• Tip the solid into a beaker and record the mass of the emptied weighing boat

• Determine the mass of solid used by calculating the difference between the two masses

• Dissolve the solid into distilled water

• Using a funnel, transfer into a 250cm3 volumetric flask, including washings

• Make up to the graduated line by carefully adding deionised water, making sure the meniscus sits on the line

• Stopper the flask and invert several times to mix the contents thoroughly

How to find the concentration of a solution of a base?

• Fill a burette with a standard solution of the acid

• Pipette a known volume of base into a conical flask 

• Add a few drops of indicator to the base in the flask 

• Add the solution of the acid from the burette until the indicator just changes colour. This is known as the end point.

• Record the volume of acid added: record your burette readings to the nearest 0.05cm3 

• Perform a rough titration and then sufficient accurate ones until the titres are concordance (within 0.10cm3 of each other)

• Calculate the mean titre from the concordance titres

Which values do you use for the mean titre?

the concordant titres (within 0.1cm³ of each other)

How to find the concentration of the base in a titration?

• Use the mean titre and the concentration of the acid to find the number of moles

• Check the ratio and divide/multiply if needed

• Use c = n/v to find the concentration using the known volume of base

How to calculate reacting quantities questions?

• Work out the moles of one substance using the given mass and the calculated Mr

• Use the balance equation to either multiply or divide depending on the value needed

• Use mass = moles x Mr to find the mass of the stated substance

How to find the percentage error 

Percentage error = uncertainty / value x 100

(The uncertainty has to be multiplied twice if two measurements are used)

Titration calculations (concentration or volume)

1. Write the balanced equation

2. Find out the moles of a substance using the concentration and volume

3. Apply it to the ratio

4. Use concentration = n/v to find the concentration or volume

In a titration which substances are placed in the conical flask and burette?

the acid (known concentration) is usually placed in the burette, with the base (known volume in the flask) 

How to find the molar mass (Mr) of an acid in titration? 

1. Write the balanced equation

2. Calculate the amount of moles of the base (in the burette) using the given concentration and volume

3. Apply this to the balanced equation to determine the amount of moles of acid

4. Scale up (if needed) to find the number of moles in the total volume 

5. Calculate the Mr using mr = mass / n 

How to find the molar mass (Mr) of a base in titration? 

1. Write the balanced equation

2. Calculate the number of moles of the acid (in the burette) using the given concentration and volume

3. Apply this to the balanced equation to determine the amount of moles of base

4. Scale up (if needed) to find the number of moles in the total volume

5. Calculate the Mr using mr = mass / n 

What are back titrations used for and method?

• Used to analyse substances that are not soluble in water but do react with acids

• Use a known mass of solid reacted with an excess of acid

• The resulting solution is titrated with a standard solution of base to determine the amount of acid left 

How to calculate back titrations?

1. Write the balanced equation for the reaction of the solid with excess of acid

2. Write the balanced equation for the reaction of the base (in burette) with the acid 

3. Calculate the moles of the base using n = v x c 

4. Use the balanced equation ration to determine the moles of acid that react with the base

5. Scale up if needed

6. Use the volume and concentration of acid to find the moles of acid at the start 

7. Take away the moles at the start from the moles of acid that react with the base to determine the moles of acid that react with the solid

8. Use the ratio from the balanced equation to determine the moles of solid

9. Mass = moles x Mr to find the mass of the solid 

What is the water of crystallisation?

• When some ionic compounds have water molecules attached to the ions in the crystal. A hydrated salt is a salt which contains water

• When hydrated salts are heated, the water is driven off and the compound is said to be anhydrous 

How to calculate the moles of water of crystallisation?

1. Find out the mass of water (mass of hydrated salt - mass anhydrous salt)

2. Work out the moles of both the anhydrous salt by calculating the mR

3. Find out the simplest whole number ratio (divide the biggest moles by the smallest, which becomes 1)

How to calculate the water of crystallisation in a carbonate?

1. Find the moles of a substance using known values 

2. Use the balanced equation to determine the moles of the carbonate 

3. Scale up if needed to find the total volume

4. Calculate the mR of the total compound by using mass / moles

5. Subtract the mR of the hydrated carbonate

6. Divide by 18 to calculate the number of moles of water

How to calculate percentage yield?

percentage yield = actual yield of product / theoretical yield of product x 100

What is the theoretical yield and how is it calculated?

• The number of moles expected in a reaction if the reactants were completely converted into products

• This can be found by calculating the number of moles of reactants used and then using the balanced equation to calculate the number of moles of product if the yield was 100%

What is the actual yield and how is it calculated?

• The actual number of moles of product formed in the reaction

• Determined by calculating the moles of the products using n = m / me 

• Or by finding a percentage of moles

What are possible reasons for low percentage yield?

• Incomplete reaction

• Side reactions

• Loss of product during transfer or purification 

How to prove that a substance is in excess?

• Work out the moles of each substance

• Use a balanced equation to determine how many moles of one substance is needed (e.g. if the ratio was 1:2, multiply the moles of the 1 substance by two to find out how many it needs)

• Show whether the actual is more than the needed, so it is in excess

What is atom economy?

A measure of the efficiency of a reaction in terms of the atoms e.g. if the atom economy is high, there is very little waste, but if it is low there is a lot of waste

Atom economy equation

Molecular mass of desired product (mR) / sum of molecular masses of all products (mR) x 100

How do the types of reaction have an effect on the atom economy?

• Addition reactions have an atom economy of 100% (e.g. adding water to a reactant to form one product)

• Substitution (swapping elements) and elimination (losing products) have an atom economy of less than 100%

Titration RP- why is it essential that the beakers used for the acid and alkali are clean and dry?

if they are wet they will alter the concentrations of the acid or alkali 

What would happen to the concentration of the acid solution if the burette was not rinsed with acid before filling?

Concentration would decrease as water in burette dilutes acid

Why does the conical flask need to be rinsed but not dried before using?

Once the pipette is used to transfer a known amount (moles) of solution, adding water would not affect the number of moles

Is it essential to clean and dry the beaker and volumetric flask before using them to make a standard solution?

No- as the solution will be made up in deionised water the beaker and flask need to be clean but not dry

Why is it essential to include the washings when making up your solution?

The traces of solution coating the beaker + stirring rod will contain some of the solid so must be transferred to the flask

How would the calculated value for the concentration differ from the actual concentration if washings were not included?

Less solute would be transferred (less moles) so calculated concentration would be higher as it assumes all mass is transferred

Relative atomic mass definition (Ar)

The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon-12

What is the relative atomic mass on the periodic table?

The atomic mass (top) number- it is the weighted average of all the isotopes

How to calculate the relative atomic mass- involving isotopes?

(Mass x abundance) + (mass x abundance) / total abundance

Relative molecular mass definition (Mr)

The mass of a molecule of the compound relative to 1/12th the mass of an atom of carbon-12

How can the relative molecular mass and relative formula mass be found? (Mr)

Adding together the relative atomic masses of all the atoms in the molecule or the formula

Relative formula mass definition

The mass of one formula unit of an ionic compound relative to 1/12th of the mass of an atom of carbon-12

Mole definition

The amount of substance that contains as many particles as there are in exactly 12g of carbon-12 (also called the Avogadro Constant)

What is the molar mass?

The mass of one mole of substance is its Mr in grams

Equation for moles in a solid

Mass (g) = molar mass (Mr) x moles (mol)

Equation for moles in a solution (dissolved in water)

Moles (mol) = vol (dm³) x conc (moldm^-3)

Equation for mass in liquid (can be used to calculate moles)

Mass (g) = density (gcm^-3) x vol (cm³)

Mg g kg tonnes conversions

Mg x 10^-3 = g g x 10^-3 = kg kg x 10^-3 = tonnes

Tonnes x 10³ = kg. kg x 10³ = g g x 10³ = mg

Equation for the number of particles/atoms in a given mass

number of particles = number of moles x Avogadro’s constant (6.02 × 10²³)

(calculate moles first using n = m/Mr)

Cm³ dm³ m³ conversions 

NOTE concentration for moles is in dm³

Cm³ × 10^-3 = dm³            dm³ × 10^-3 = m³ 

M³ × 10³ = dm³     dm³ × 10³ = cm³

What is the ideal gas equation?

pV = nRT

p = pressure in Pa

v = volume in m³

n = number of moles

R = gas constant (given in Q)

T = temperature (C + 273 = K)

Density equation

mass (g) = density x volume (cm³)

Empirical formula definition

The simplest whole number ratio of atoms of each element present in a compound

How to calculate empirical formula 

Convert mass to mole (find the moles by using Mr and percentage mass if given)

Divide the biggest number by the smallest, so the smallest is 1

Express as a ratio and multiply till whole

Common fractions for empirical formula: 0.25, 0.5, 0.,33, 0.75, 0.67

0.25 = x4

0.5 = x2

0.33 = x3

0.75 = x4

0.67 ×3

Molecular formula definition

The actual number of atoms of each element present in a molecule

How to calculate molecular formula

Calculate empirical formula

Calculate relative mass of the empirical formula (if H20, = 2+16)

Use the Mr (given in Q) to calculate how many times bigger the molecular formula is (give it by relative mass) compared to empirical formula