Amounts of Substance
In this article we will be looking at the how, in Chemistry, we calculate and represent amounts of substance and chemical reactions quantitively.
RAM and RFM
RAM stands for relative atomic mass (Ar) and this means the average mass of an atom, relative to 1/12 of carbon-12 . Which effectively is the same as the mass number but averaged out. So that the RAM of chlorine is 35.5 because 25% has a mass of 37 and 75% a mass of 35. This is calculated using a mass spectrometer.
RFM stands for relative formula mass (Mr)this is the sum of the RAMs of all the elements in a molecule. I.e. it is the mass of the molecule. These two measures are used in Chemistry because it is more accurate.
The mole is a unit of molecules (where it gets its name) to help chemists. A mole is 6.02 x 1023 atoms/molecules; this number is known as Avogadro's constant. A mole of any substance has the mass of the its RAM/RFM, meaning that calculations can easily be made; and this is the main use of the mole. The following are some examples of questions to help you understand how to use the mole.
|What is the mass of 1 mole of chlorine?
|What is the mass of 0.5 moles of lead nitrate, Pb(NO3)2?
0.5 x [207 2(14+48)]
|What is the mass of 5 moles of carbon?
=5 x 12
|How many moles are there in 4g of bromine?
20 / 80
|How many moles are there in 6.4g of sulphur dioxide?
(32 + 16 + 16) / 6.4
Calculating concentrations is very simple. You use the following folmula:
The key to getting to grips with moles is to do lots of practice exercises; you should be able to find these in your textbook or you may wish to get a practice book with lots of exercises in it.
Ideal Gas Equation
The ideal gas equation is an equation that links pressure, volume, moles, the gas constant and temperature, for calculations. Here it is:
The units you use to do the calculation are very specific, and sometimes requires converting to get it to work, below are some explanations of what they are and how to use them.
The pressure is measured in Pascals, this is the same as Newtons per metre squared.
Volume is measured in metres cubed m3.
The gas constant is 8.31 this number does not change regardless of what the other numbers are.
The temperature is measured in kelvin this is a scale of temperature where 0K is absolute zero and it is impossible to get a negative value on the kelvin scale. 273.15K is 0oC.
Types of Formulae
There are two types of formula that we use. The first is the empirical formula, which is the basic ratio of molecules to each other in its simplest form. You can calculate this as follows:
Find the empirical formula of a compund where, by mass C = 85.7%, H=14.3%.
Convert the percentages to mass in terms of 100g (85.7g 14.3g). Then you divide the masses by the Ar values.
C = 7.1, H = 14.3
Now you must divide by the smallest number in this case it is 7.1
So the empirical formula is CH2.
The second type of formula is molecular formula. This is the actual number of atoms in the molecule.