Purity & Separating Mixtures (OCR GCSE Combined Science A (Gateway): Chemistry): Flashcards

Relative formula mass (Mr) is the total mass of a molecule, found by adding up the relative atomic masses of all the atoms in its formula.

It is measured relative to 1/12 of the mass of one atom of carbon-12.

The relative atomic mass is the larger of the two numbers shown for each element on the periodic table.

The smaller number is the atomic number (number of protons).

True.

Isotopes of an element have the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers.

Mr of H₂O = (2 × 1) + 16 = 18

Add up the Ar of every atom in the formula: 2 hydrogen atoms and 1 oxygen atom.

To calculate the Mr of a substance, add up the relative atomic masses of all the atoms present in the formula.

Mr of CO₂ = 12 + (2 × 16) = 44

Count each atom in the formula and multiply by its Ar before adding.

In a balanced equation, the sum of the relative formula masses of the reactants equals the sum of the relative formula masses of the products.

Mass is neither created nor destroyed during a chemical reaction.

False.

The Mr of NaCl = 23 + 35.5 = 58.5. To calculate the relative formula mass, you add up the relative atomic masses of all the atoms in the formula, not just one of them.

An empirical formula is the simplest whole number ratio of atoms of each element in a compound.

For example, the empirical formula of hexane (C₆H₁₄) is C₃H₇.

The molecular formula gives the actual number of atoms of each element in a molecule.

The empirical formula gives the simplest whole number ratio of atoms. Sometimes they are the same, for example CO₂.

The empirical formula is C₃H₇.

Divide the number of each atom by the highest common factor — in this case 2 — to get the simplest ratio.

True.

The empirical formula shows the simplest whole-number ratio of atoms in a compound. The molecular formula shows the actual number of each type of atom in a molecule. For example, ethene has empirical formula CH₂ and molecular formula C₂H₄.

Identify the ions present in the lattice, then balance their charges so the overall charge is zero.

For example, Na⁺ and Cl^- each have a charge of ±1, so the empirical formula is NaCl.

The empirical formula of a compound is the simplest whole number ratio of atoms of each element.

The equation for percentage by mass is:

% mass = (Ar × number of atoms of that element ÷ Mr of the compound) × 100

where Ar is the relative atomic mass and Mr is the relative formula mass.

Mr of CaCO₃ = 40 + 12 + (3 × 16) = 100

% mass of Ca = (40 ÷ 100) × 100 = 40%

False.

A dot-and-cross diagram shows the outer shell electrons of bonded atoms using dots and crosses. For ionic compounds, it shows the transfer of electrons and the resulting ionic charges on each ion.

An alloy is a mixture of metals, often with small amounts of other elements added.

Alloys are designed to be stronger and harder than the pure metals they contain.

Alloys contain atoms of different sizes, which distorts the regular arrangement of atoms.

This makes it harder for the layers to slide over each other, increasing the hardness of the material.

True.

Brass is a common example of an alloy, containing 70% copper and 30% zinc. An alloy is a mixture of metals, designed to have improved properties such as greater hardness.

A formulation is a mixture that has been deliberately designed as a useful product.

Each component is present in carefully measured quantities so that it contributes the required properties. Examples include paints, medicines and food.

Alloys are harder than pure metals because they contain atoms of different sizes, which distorts the regular arrangement of layers.

1. Paint

2. Medicines

3. Food

All are complex mixtures made to an exact recipe where each component provides a specific property.

An alloy is a formulation because it is a mixture that has been designed to have specific properties by combining metals (or metals with non-metals) in precise proportions. The composition is carefully controlled to achieve the desired strength, hardness or other characteristics.

An alloy is a formulation because it is a mixture deliberately designed as a useful product.

Each metal is present in a carefully measured proportion so that the alloy has the required properties — it is not a random mixture.

Filtration is used to separate an insoluble solid from a liquid or solution.

For example, sand can be separated from a mixture of sand and water. Solid particles are too large to pass through the filter paper and remain as the residue.

Crystallisation is a separation technique used to obtain a dissolved solid from a solution by evaporating the solvent to form a saturated solution, then cooling slowly so crystals grow.

False.

Simple distillation separates a soluble solid from a liquid (or a pure liquid from a mixture). Separating two miscible liquids requires fractional distillation, which uses a fractionating column.

The salt water is heated until the water evaporates and rises as vapour.

The vapour passes through a condenser where it cools and condenses into pure liquid water, collected in a beaker. The salt is left behind in the flask.

Fractional distillation works by heating a mixture to the boiling point of the substance with the lowest boiling point, which evaporates first and is collected by condensation.

The fractionating column contains glass beads that increase the surface area available inside the column.

Substances with higher boiling points condense on the beads and return to the flask, while only the substance with the lowest boiling point reaches the condenser and is collected.

1. Filtration to remove the sand (insoluble solid) from the water and dissolved salt

2. Crystallisation to separate the salt (dissolved solid) from the water

Filtration must come first because the sand must be removed before the salt can be crystallised.

True.

To test if a solution is saturated, you dip a clean, dry, cold glass rod into the hot solution. If the solution is saturated, crystals will form on the glass rod as it cools the solution locally.

Miscible liquids are liquids that mix completely with each other to form a single solution.

Fractional distillation is used to separate miscible liquids, such as ethanol and water, by exploiting their different boiling points.

Heating should be stopped when the temperature starts to rise above 78 °C (the boiling point of ethanol) towards 100 °C.

At 78 °C the ethanol boils and distils out of the mixture. If heating continues to 100 °C, the water will also begin to evaporate and contaminate the collected ethanol.

Crystallisation is used to separate a dissolved solid from a solution, where the solid is much more soluble in hot solvent than in cold.

The mobile phase is the liquid solvent that moves up the paper, carrying dissolved substances with it.

The stationary phase is the chromatography paper itself, which does not move.

Ink from a pen would dissolve in the solvent and run up the paper along with the samples, contaminating the chromatogram.

Pencil is insoluble in the solvent, so it stays in place as a fixed baseline.

Substances have different solubilities in the solvent, so those with higher solubility spend more time in the mobile phase and are carried further up the paper.

Substances also have different affinities for the stationary phase, which slows them down by different amounts.

True.

A pure substance contains only one type of particle (element or compound) and has a sharp melting point. A mixture contains two or more different substances not chemically combined, and melts over a range of temperatures.

An Rf value (retention factor) is the ratio of the distance moved by a substance to the distance moved by the solvent front, both measured from the baseline.

Rf = distance moved by compound ÷ distance moved by solvent. It has no units.

The Rf value of a substance is always the same when the solvent is the same, but changes if the solvent is changed.

In gas chromatography, the mobile phase is an unreactive carrier gas (such as nitrogen) rather than a liquid solvent.

The stationary phase is an unreactive solid (such as silica) packed inside a long column, rather than paper.

1. The number of peaks shows how many compounds are present

2. The height of each peak shows the relative amount of each compound

3. The position of each peak (retention time) indicates how strongly the compound interacts with the stationary phase

False.

In thin-layer chromatography (TLC), the stationary phase is a thin layer of an inert substance such as silica, not chromatography paper. Chromatography paper is the stationary phase in paper chromatography.

Rf = 4.5 ÷ 9.0 = 0.5

Rf values are always between 0 and 1 and have no units. This value can be compared to reference Rf values to help identify the compound.