Slides in this set
· One mole of atoms or molecules of any substance will
have a mass in grams equal to the relative formula
mass (Ar or Mr) for that substance. E.g. carbon has a Ar
or 12 so one mole of carbon weighs 12g.
· Molar mass is another way of saying mass of one mole.
It is measured in grams per mole (g/mol).
· Number of moles = mass in g / Mr.
· The relative atomic mass of an element is the average
mass of an atom of the element compared to the mass
of 1/12th of an atom of carbon-12.…read more
Reacting Masses and Empirical
· You can use moles to calculate masses in reactions. First you write out a balanced
equation, then calculate the number of moles in the mass you have been given. Then
look at the ratio of moles in the equation. Finally calculate the mass of moles in what
you are trying to find out.
· Percentage composition by mass = Ar x no. of atoms of element / Mr of whole
· Empirical formulas give the smallest whole number ratio of atoms in a compound.
You can work it out by following simple steps.
· 1. List the elements
· 2. Write in the experimental masses or percentages
· 3. Divide by the Ar for each element
· 4. Turn the numbers you get into a simple ratio by multiplying and/or dividing them
· 5. Get its ratio in the simplest form, and that's the empirical formula.
e.g. the chemical formula for ethane is CH and the empirical formula is CH…read more
· A concentration of a substance is measured in
moles per dm³ or grams per dm³.
· The more solute you dissolve in a given volume,
the more crowded the solute molecules are and
the more concentrated the solution.
· Concentration= number of moles / volume
· If they ask for the concentration is grams per dm³
then use this formula:
· Number of moles= mass / relative formula mass
· If asked to dilute a solution then follow these steps:
· 1. Work out the ratio of the two concentrations- divide the small number
by the big number.
· 2. Multiply this ratio by the volume of solution you want to end up with-
tells you how much original acid you want to dilute. Volume to dilute=
ratio x total volume
· 3. Work out the volume of water you'll need. Volume of water= total
volume- volume to dilute
· Food packaging gives GDA- these amounts may be inaccurate as you may
eat more/less or you may add other things (milk to cereal= more calcium).
· You can use sodium content to estimate the amount of salt. Find the ratio
of sodium chloride's Mr to sodium's Ar. Multiply this by the amount of
· This amount of sodium present won't all come from salt- it may come
from sodium nitrate which is used as a preservative so this amount is
normally an overestimate.…read more
· Titrations allow you to find out exactly how much acid
or alkali is needed to neutralise a quantity or acid or
· You need to get several consistent readings to increase
the accuracy of your titration and to spot any
anomalous readings. The first titration should be a
rough test to get an approximate idea of the colour
change, then you repeat the test making sure you get
roughly the same results each time.
· Universal indicator is used to see a gradual colour
change from acid to alkali but you want to see the
sudden colour change so a single indicator is used.…read more