c2.1-Purity and Separating Mixtures Flashcards
c2.1.1-What is relative and atomic formula mass?
Relative atomic mass(Ar) is the mean mass of an atom of an element compared to 1/12 the mass of a 12|6 Carbon atom. Relative formula mass(Mr) is mean mass of a unit of a substance compared to 1/12 the mass of a 12|6 Carbon atom.
Relative formula mass(Mr) = Ar values X atoms and add them together
c2.1.2-How do you calculate an empirical formula?
Find HCF and divide chemical formula by HCF. C4H10 Eg: 4/2=2, 10/2=5 making C2H5.
For diagrams, work out number of atoms at each element. Eg: C=2, H=4, O=2. 2/2=1, 4/2=2, 2/2=1 making CH2O
c2.1.3-What does pure mean?
Pure describes natural substances that have not been processed or changed. A pure substance consists of just one element or compound. This means mixtures are impure substances as they contain more than one element or compound.
c2.1.3-How do you use melting points to determine purity?
The melting point of a substance is the temperature at which it changes from solid to liquid state. If a substance is impure, it’s melting point is less than that of a pure substance and often melts over a range of temperatures. You can determine melting point by heating the substance slowly and stirring it as it melts.
c2.1.4-What happens when a substance dissolves?
A solution forms when one substance dissolves in another. The solute is the substance that dissolves and the solvent is the substance it dissolves in. When a substance dissolves, it’s particles separate and become mixed. If a substance can dissolve, it’s soluble and if it can’t dissolve, it is insoluble.
c2.1.4-How does filtration work?
Filtration separates an insoluble substance in the solid state from substances in the liquid state. It works because filter paper has tiny holes and when you filter sand and water, water molecules are small enough to pass through. The sand stays behind in the filter as residue and the water passes through as the filtrate.
c2.1.4-How does crystallisation work?
Crystallisation is when you need to heat the solution gently until it becomes a saturated solution. A solution is saturated when no more solute can be dissolved and crystals start forming so you let the solution cool. As it cools, the solubility of the solute decreases and more crystals form. You separate the rest with filtration.
c2.1.5-How does simple distillation work?
Simple distillation separates a solvent from a solution and relies on the solvent having a lower boiling point than the solute. When the solution is heated, the solvent boils while the solute doesn’t and the solvent escapes in its gas state. It is then cooled and condensed back to its liquid state by a condenser, a piece of apparatus which is kept cold by a flow of cold water.
c2.1.5-How does fractional distillation work?
Fractional distillation separates two or more substances from a mixture in liquid state. It relies on each substance having a different boiling point and using a fractionating column. In the fractionating column, the vapours on the inside condense heating it up, ethanol vapour can’t condense when the temperature reaches it and ethanol vapour passes into the condenser.
c2.1.6-How does chromatography work?
Chromatography relies on chemical phases in a stationary phase that doesn’t move and a mobile phase that does. In paper chromatography, the stationary phase is absorbent paper and the mobile phase is a solvent in the liquid state.
c2.1.6-How does thin-layer chromatography work?
1)Put the solvent into a chromatography tank. If the solvent is flammable, make sure there are no naked flames.
2)Add a small sample amount to the baseline, making sure not to damage powder on the plate.
3)Let the solvent travel though the powder and take the plate out before it reaches the top
4)Analyse the coloured spot patterns which is called a chromatogram.
c2.1.6-How does gas chromatography work?
Gas chromatography separates components of a mixture and measures their amounts. The stationary phase is silica or alumina power packed into a metal column. The mobile phase is an unreactive carrier gas such as nitrogen which doesn’t react with the sample.
c2.1.6-How do you calculate an Rf value?
Rf Value = distance travelled by substance / distance travelled by solvent
c2.1.7-What are the advantages of thin-layer chromatography over paper chromatography?
Thin-layer chromatography is quicker, more sensitive so a smaller sample can be used and there is a larger range of stationary phases and solvents to choose from.
c2.1.7-How to choose a separation method?
Insoluble and soluble substances are dissolved by filtration. A solute dissolved in a solvent is separated by crystallisation for the solute and simple distillation for the solvent. Two or more substances in the liquid state are separated by fractional distillation and coloured soluble substances are separated by paper and thin-layer chromatography.
