What does the Rf value of a substance tell you?

It tells you how far the substance travelled compared with the solvent, on a scale from 0 to 1.

From where are the two distances measured when an Rf value is worked out?

Both are measured from the baseline, one to the centre of the spot and one to the solvent front.

A spot moves 3.0 cm while the solvent moves 12.0 cm. Calculate the Rf value.

It is 0.25, because 3.0 ÷ 12.0 = 0.25.

A substance has an Rf value of 0.60 and the solvent front moved 10.0 cm. How far did the spot move?

It moved 6.0 cm, because 0.60 × 10.0 = 6.0.

A spot travelled 4.5 cm and the solvent travelled 9.0 cm. Give the Rf value, and state how far the spot would have moved if the solvent front had reached only 6.0 cm.

The Rf value is 0.50 because 4.5 ÷ 9.0 = 0.50, and the spot would have moved 3.0 cm because 0.50 × 6.0 = 3.0.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions State symbols in equations Conservation of mass, and changes of mass in open systems Exothermic and endothermic reactions, and reaction profiles Activation energy and why reactions need it Oxidation and reduction in terms of oxygen Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids The pH scale, acidity and alkalinity Measuring pH with indicators and pH meters Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Life cycle assessment Recycling, and the decisions behind it Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine

Elements, Compounds and Mixtures

Interpreting chromatograms and Rf values

3 things to know
Paper 3 (Chemistry)

Overview

What an Rf value means, the formula and where the distances are measured from, worked Rf calculations, why Rf never exceeds 1, why the solvent must match, and reading purity and identity from a chromatogram.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the formula used to calculate an Rf value.

Key term

Why can an Rf value never be greater than 1?

Key term

Why must the same solvent be used when an Rf value is compared with a published value?

Questions from this subtopic

Each one with the model answer underneath.

1
What does the Rf value of a substance tell you?
Model answer
It tells you how far the substance travelled compared with the solvent, on a scale from 0 to 1.
2
State the formula used to calculate an Rf value.
Model answer
Rf equals the distance moved by the substance divided by the distance moved by the solvent.
3
From where are the two distances measured when an Rf value is worked out?
Model answer
Both are measured from the baseline, one to the centre of the spot and one to the solvent front.
4
A spot moves 3.0 cm while the solvent moves 12.0 cm. Calculate the Rf value.
Model answer
It is 0.25, because 3.0 ÷ 12.0 = 0.25.
5
A substance has an Rf value of 0.60 and the solvent front moved 10.0 cm. How far did the spot move?
Model answer
It moved 6.0 cm, because 0.60 × 10.0 = 6.0.
6
A spot travelled 4.5 cm and the solvent travelled 9.0 cm. Give the Rf value, and state how far the spot would have moved if the solvent front had reached only 6.0 cm.
Model answer
The Rf value is 0.50 because 4.5 ÷ 9.0 = 0.50, and the spot would have moved 3.0 cm because 0.50 × 6.0 = 3.0.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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