Amount of Substance (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define the mole
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Cards in this collection (32)
Define the mole
The mole is the SI unit of amount of substance, defined as the quantity of particles equal to the Avogadro constant (6.02 × 1023 mol-1).
True or False?
One mole of any gas occupies 24.0 dm3 at room temperature and pressure.
True.
At RTP, the molar gas volume of any gas is 24.0 dm3 mol-1, regardless of the identity of the gas.
What is the relationship between amount of substance, mass and molar mass?
Amount of substance is related to mass and molar mass by n = m/M, where n is moles, m is mass in grams and M is molar mass in g mol-1.
The Avogadro constant, NA, has the value .......... mol-1 and defines the number of particles in .......... of any substance.
The Avogadro constant, NA, has the value 6.02 × 1023 mol-1 and defines the number of particles in one mole of any substance.
How many moles are present in 9.6 dm3 of chlorine gas at RTP?
9.6 dm3 of chlorine at RTP contains 0.4 mol, calculated by dividing the volume (9.6 dm3) by the molar gas volume (24.0 dm3 mol-1).
True or False?
The Avogadro constant applies only to atoms.
False.
The Avogadro constant applies to any type of particle, including atoms, molecules, ions and electrons.
Define empirical formula
The empirical formula is the simplest whole number ratio of the atoms of each element present in a compound.
True or False?
The molecular formula of a compound is always different from its empirical formula.
False.
The molecular formula and empirical formula are the same when the ratio of atoms in the molecule is already in its simplest whole number form, as in CH4 or H2O.
To calculate the empirical formula from percentage composition, treat the percentage values as .......... in grams, then divide each by the element's .......... to get moles.
To calculate the empirical formula from percentage composition, treat the percentage values as masses in grams, then divide each by the element's relative atomic mass to get moles.
How do you calculate the molecular formula from the empirical formula?
Divide the compound's relative molecular mass by the relative mass of the empirical formula to get a whole number ratio, then multiply the subscripts in the empirical formula by that ratio.
Define water of crystallisation
Water of crystallisation refers to water molecules chemically bound within the crystal structure of a hydrated salt, shown by a dot in the formula (e.g. CuSO4•5H2O).
True or False?
Heating a hydrated salt converts it to its anhydrous form.
True.
Heating a hydrated salt drives off the water of crystallisation, leaving the anhydrous compound behind.
How is the number of moles of water of crystallisation determined experimentally?
The hydrated salt is weighed before and after heating to constant mass; the mass of water lost is divided by 18.0 g mol-1 and compared to the moles of anhydrous salt to find the ratio.
Define stoichiometry
Stoichiometry is the mole ratios between reactants and products in a balanced chemical equation, used to calculate unknown masses, moles or volumes.
True or False?
The mole ratio in a balanced equation can be used directly to calculate reacting masses.
True.
Once the moles of one substance are known, the mole ratio from the balanced equation is applied to find the moles of any other substance, which is then converted to mass using molar mass.
What equation links concentration, moles and volume of a solution?
Concentration (mol dm-3) is linked to moles and volume by concentration = moles ÷ volume, where volume must be in dm3.
At room temperature and pressure, one mole of any gas occupies .......... dm3. This is known as the .......... .
At room temperature and pressure, one mole of any gas occupies 24.0 dm3. This is known as the molar gas volume.
What does Avogadro's hypothesis state about equal volumes of gases?
Avogadro's hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, regardless of the identity of the gas.
True or False?
To convert cm3 to dm3, you divide by 1000.
True.
There are 1000 cm3 in 1 dm3, so dividing a volume in cm3 by 1000 converts it to dm3 for use in concentration calculations.
Define ideal gas
An ideal gas is a theoretical gas whose molecules have negligible volume, no intermolecular forces between them and undergo perfectly elastic collisions.
True or False?
Real gases deviate most from ideal behaviour at high temperature and low pressure.
False.
Real gases deviate most from ideal behaviour at low temperatures and high pressures, where intermolecular forces become significant and the volume of molecules is no longer negligible.
State the ideal gas equation and define each term.
The ideal gas equation is pV = nRT, where p is pressure (Pa), V is volume (m3), n is moles (mol), R is the gas constant (8.314 J mol-1 K-1) and T is temperature (K).
When using the ideal gas equation, temperature must be in .......... and pressure must be in .......... .
When using the ideal gas equation, temperature must be in kelvin and pressure must be in pascals.
Why does the volume of a gas increase when it is heated at constant pressure?
Heating gives particles greater kinetic energy, causing more frequent collisions with container walls. To maintain constant pressure the gas must expand, so volume increases.
True or False?
To convert a temperature from °C to kelvin, you add 273.
True.
The conversion from degrees Celsius to kelvin is T (K) = T (°C) + 273, ensuring temperature is expressed on the absolute scale required by the ideal gas equation.
Define percentage yield
Percentage yield is the ratio of the actual yield obtained experimentally to the theoretical yield predicted from the balanced equation, expressed as a percentage.
True or False?
A percentage yield greater than 100% is theoretically possible if impurities are present in the product.
False.
Percentage yield cannot exceed 100% in a valid experiment; an apparent yield above 100% indicates the product is impure or an error has been made in the calculation.
Give three reasons why the percentage yield of a reaction may be less than 100%.
The reaction may not reach completion
Side reactions may consume reactants
Product may be lost during separation or purification
Atom economy = .......... ÷ .......... × 100
Atom economy = molecular mass of desired product ÷ total molecular mass of all products × 100
Why do addition reactions always have 100% atom economy?
Addition reactions produce only one product, so all atoms from the reactants are incorporated into the desired product and no by-products are formed.
Define atom economy
Atom economy is a measure of what proportion of the total mass of reactants ends up in the desired product, expressed as a percentage.
True or False?
A reaction with a high atom economy reduces waste and is more sustainable.
True.
High atom economy means fewer atoms are wasted as by-products, reducing raw material consumption and making the process more environmentally sustainable.
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