Amount of substance | OCR A Level Biology

Define the mole.

The amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12.

Define relative atomic mass.

The average mass of one atom compared to one twelfth of the mass of one atom of carbon-12.

What is molar mass?

The mass in grams of 1 mole of a substance, measured in g mol-1.

What is the molar gas volume at room temperature (25°C) and pressure (1atm)?

24 dm3 mol-1.

What is Avogadro's constant?

6.02 x 10^23, representing the number of entities in one mole of a substance.

What is the formula to calculate the amount in moles from mass and molar mass?

Amount (mol) = mass (g) / molar mass (Mr).

How many milligrams are in 1 gram?

1000 mg.

How many grams are in 1 tonne?

1,000,000 g (or 1000 kg).

What rule should be followed regarding significant figures in calculations?

Use the same number of significant figures as the data provided in the question; if mixed, use the smallest number.

Define empirical formula.

The simplest ratio of atoms of each element in a compound.

Define molecular formula.

The actual number of atoms of each element in a compound.

How do you calculate a molecular formula from an empirical formula?

Divide the relative molecular mass (Mr) by the mass of the empirical formula to find the multiplier, then multiply the empirical formula subscripts by that number.

What is a hydrated salt?

A salt that contains water of crystallisation within its structure.

Why is a lid used when heating a crucible to determine water of crystallisation?

To prevent the loss of solid while allowing water vapour to escape.

Why is it important to heat a crucible until a constant mass is reached?

To ensure that the reaction (e.g., removal of water) is complete.

What is the potential error if a crucible is wet before starting an experiment?

The mass loss would be recorded as too large because the water from the crucible itself would evaporate.

What is the formula for molar concentration?

Concentration (mol dm-3) = amount (mol) / volume (dm3).

How do you convert cm3 to dm3?

Divide by 1000.

How do you convert m3 to dm3?

Multiply by 1000.

What is the relationship between 1 dm3 and 1 litre?

They are equivalent (1 dm3 = 1 L).

How do you convert concentration from mol dm-3 to g dm-3?

Multiply the concentration in mol dm-3 by the molar mass (Mr) of the substance.

What is mass concentration?

The mass of solute dissolved per unit volume of solution, measured in g dm-3.

What happens when soluble ionic solids dissolve in water?

They dissociate into separate ions.

What is the unit for molar concentration?

mol dm-3 or M.

What is the unit for molar mass?

g mol-1.

What is the unit of molar concentration?

mol dm-3 or M

How do you convert dm3 to m3?

Divide by 1000

How do you convert cm3 to m3?

Divide by 1,000,000

What is the relationship between 1 cm3 and 1 mL?

1 cm3 = 1 mL

What is the formula for amount in moles using mass and molar mass?

amount = mass / Mr

How do you calculate concentration in g dm-3 from mol dm-3?

Multiply the concentration in mol dm-3 by the Mr of the substance

What happens to ionic solids when they dissolve in water?

They dissociate into separate ions

Why is a volumetric pipette more accurate than a measuring cylinder?

It has a smaller uncertainty

What is the formula for calculating the concentration of a diluted solution?

Original volume x original concentration = new diluted volume x new diluted concentration

What happens to the amount of moles of solute when a solution is diluted?

The amount of moles remains unchanged

What is the ideal gas equation?

PV = nRT

What are the required units for the ideal gas equation (PV = nRT)?

Pressure in Pa, Volume in m3, Temperature in K

How do you convert degrees Celsius to Kelvin?

Add 273

What is the value of the gas constant R?

8.31 JK-1mol-1

What safety precaution should be taken for irritant substances?

Wear goggles

What safety precaution should be taken for toxic substances?

Wear gloves, avoid skin contact, and wash hands after use

How should you fill a volumetric flask to the mark?

Use a dropping pipette for the last few drops so the bottom of the meniscus sits on the line

What is the purpose of inverting a volumetric flask?

To ensure a uniform solution

What does the 'n' represent in the ideal gas equation for a mixture of gases?

The total moles of all gases in the mixture

How can you calculate the molar mass (Mr) of a volatile liquid using the ideal gas equation?

Calculate moles using n = PV/RT, then use Mr = mass / amount

What is the molar gas volume at room temperature and pressure?

1 mol of gas = 24 dm3

What should be noted when recording the volume of a gas in a gas syringe?

The temperature and pressure of the room

What is the unit of mass concentration?

g dm-3

What are three common errors when using a gas syringe?

Gas escaping before the bung is inserted, the syringe sticking, and gas being soluble in water.

How can you calculate the pressure of a gas mixture if the temperature remains constant?

Use the formula P2 = (P1 * V1) / V2.

What is the molar volume of any gas at room temperature (25°C) and pressure (1 atm)?

24 dm3 per mole.

What does Avogadro's Law state regarding reacting volumes of gases?

Equal volumes of any gases at the same temperature and pressure contain equal numbers of molecules.

What is the value of Avogadro's constant?

6.02 x 10^23 particles per mole.

How do you calculate the number of particles in a substance?

Number of particles = amount in moles * Avogadro's constant.

How do you calculate density?

Density = mass / volume.

What are the three steps to convert quantities between different substances using a balanced equation?

1. Convert given quantity to moles, 2. Use the balanced equation mole ratio to find moles of the second substance, 3. Convert moles of the second substance to the required unit.

What is a limiting reactant?

The reactant that is completely consumed in a reaction and determines the maximum amount of product that can be formed.

How do you determine which reactant is in excess?

Calculate the moles of each reactant and compare them to the stoichiometric ratio provided by the balanced equation.

What is the formula for percentage yield?

(Actual yield / Theoretical yield) * 100.

List three reasons why percentage yield might be less than 100%.

Incomplete reactions, side reactions, and losses during transfer or purification.

What is the formula for percentage atom economy?

(Mass of useful products / Mass of all reactants) * 100.

Why is high atom economy important in sustainable chemistry?

It minimizes the production of waste products.

What is the atom economy of a reaction with only one product?

100%.

How can the economics of a process with a waste product be improved?

By selling the by-product for other uses.

What is the relationship between moles and mass?

Amount in moles = mass / molar mass (Mr).

How do you calculate the amount of gas in moles using the ideal gas equation?

n = PV / RT.

When calculating the number of ions in a solution, what must you consider regarding the formula?

The number of ions produced per mole of the compound (e.g., MgCl2 produces two chloride ions per mole).

What unit is typically used for density in these calculations?

g cm^-3 or kg dm^-3.

Does the mole ratio in a balanced equation apply to gas volumes?

Yes, provided the gases are measured under the same conditions of temperature and pressure.

What is the definition of a mole?

The amount of substance that contains the same number of particles as there are atoms in 12 grams of carbon-12.

If you have 500 cm^3 of methane reacting with oxygen, how do you find the volume of oxygen needed?

Use the stoichiometric ratio from the balanced equation (e.g., if the ratio is 1:2, multiply the methane volume by 2).

What is the purpose of a catalytic converter reaction?

To convert harmful gases like CO and NO into less harmful gases like CO2 and N2.

How do you calculate the mass of a substance if you know the density and volume?

Mass = density * volume.

When calculating atom economy, should you include balancing numbers?

Yes, balancing numbers must be included in the calculation.

What is the difference between actual yield and theoretical yield?

Actual yield is the amount of product obtained experimentally, while theoretical yield is the maximum amount calculated from stoichiometry.