The following came out a UCLAN Scitech discussion on the use of 1M to mean 1 mol dm-³ ie one mole of a substance in one cubic decimetre of solution and is the contribution of Dr Peter Borrows.
'There are various internationally agreed conventions about units and scientific terminology. Mostly in the UK widespread adoption of these datesback to the 1970s and the work of the Metrication Board.The metric system as we call it now originated in Napoleonic France. It was intended that 1000 g of water should have a volume of 1 litre -the same as one cubic decimetre and that litres would be used for fluids but cubic centimetres (cm³) for solids.
However, for convenience, and following practice dating back to the Middle Ages, a standard metre, a standard kilogram, etc were constructed. (The standard metre was the distance between 2 marks on a platinum rod, the kilogram was a lump of platinum, etc). Unfortunately, as the 19th century progressed, and measurements were made with increasing accuracy, a problem arose. It was discovered that the standard litre did not weigh exactly 1 kg. In 1904 an international conference agreed to redefine the litre as the volume occupied by 1 kg of water under certain specified conditions. This resulted in the rather inconvenient relationship:- 1 litre = 1.000 028 cubic dm. This persisted until 1964, when a further international conference adopted a revised definition in which 1 litre = 1 cubic dm (exactly) which, of course, is what everybody had thought in the first place. Or nearly everybody. Given the accuracy with which pupils work and indeed almost everybody else, there was no practical difference, but the litre was NOT to be used for accurate work and hence, generally, was not felt suitable for scientific work. There is another reason for avoiding litres - the abbreviation for it, l, looks like the numeral one in many fonts. You could avoid this by doing what the Americans sometimes do and use a capital letter instead, L. However, that is definitely wrong by the internationally agreed conventions: abbreviations for units shall begin with lower case letters (g, m) unless the unit is based on the name of a person, in which case capital letters areto be used (W, N, etc). Despite a spoof some years ago, there was never a Jean-Baptiste Litre, after whom the unit was named. Instead, the word litre was based on an obscure ancient French unit of capacity, litron, which in turn was ultimately derived from a Greek word meaning pound (which in turn is why the UK currency symbol is based on the letter L). A further reason for preferring volumes and other quantities measured in the seven base units of the SI system (the metre, kilogram, second, ampere, kelvin, mole and candela) is that these form a coherent system, that is to say if you put data in using SI units, you get the answer out in SI units.It is relatively simple and obvious from the name of the unit how you convert cm into metres and hence cubic cm into cubic m. The relationship with litres is not so self-evident. In the good (bad?) old days it was a nightmare trying to remember relevant conversion factors if you were working in ergs or dynes or foot-poundals ... Under the same international conventions, the word 'molar' means 'divided by the amount (in moles)' molar mass is measured in kg per mole. All of this means that you can't have the word molar by itself. So you can't have a 2 molar solution. There is a further point, from a teaching point of view, that 2M means a concentration, 2 mol dm-³ and by just writing it as '2M' doesn't actually help the students very much. (In advanced work, people havesometimes referred to a 2 molal solution. Does that mean anything to most technicians?
'M' was a convenient abbreviation but it is not self-evident what it means. Doubtless, busy technicians will continue to use the abbreviation but it shouldn't be used in exams so it's probably not a good idea to use it for pupils who are preparing for exams.
'There are various internationally agreed conventions about units and scientific terminology. Mostly in the UK widespread adoption of these datesback to the 1970s and the work of the Metrication Board.The metric system as we call it now originated in Napoleonic France. It was intended that 1000 g of water should have a volume of 1 litre -the same as one cubic decimetre and that litres would be used for fluids but cubic centimetres (cm³) for solids.
However, for convenience, and following practice dating back to the Middle Ages, a standard metre, a standard kilogram, etc were constructed. (The standard metre was the distance between 2 marks on a platinum rod, the kilogram was a lump of platinum, etc). Unfortunately, as the 19th century progressed, and measurements were made with increasing accuracy, a problem arose. It was discovered that the standard litre did not weigh exactly 1 kg. In 1904 an international conference agreed to redefine the litre as the volume occupied by 1 kg of water under certain specified conditions. This resulted in the rather inconvenient relationship:- 1 litre = 1.000 028 cubic dm. This persisted until 1964, when a further international conference adopted a revised definition in which 1 litre = 1 cubic dm (exactly) which, of course, is what everybody had thought in the first place. Or nearly everybody. Given the accuracy with which pupils work and indeed almost everybody else, there was no practical difference, but the litre was NOT to be used for accurate work and hence, generally, was not felt suitable for scientific work. There is another reason for avoiding litres - the abbreviation for it, l, looks like the numeral one in many fonts. You could avoid this by doing what the Americans sometimes do and use a capital letter instead, L. However, that is definitely wrong by the internationally agreed conventions: abbreviations for units shall begin with lower case letters (g, m) unless the unit is based on the name of a person, in which case capital letters areto be used (W, N, etc). Despite a spoof some years ago, there was never a Jean-Baptiste Litre, after whom the unit was named. Instead, the word litre was based on an obscure ancient French unit of capacity, litron, which in turn was ultimately derived from a Greek word meaning pound (which in turn is why the UK currency symbol is based on the letter L). A further reason for preferring volumes and other quantities measured in the seven base units of the SI system (the metre, kilogram, second, ampere, kelvin, mole and candela) is that these form a coherent system, that is to say if you put data in using SI units, you get the answer out in SI units.It is relatively simple and obvious from the name of the unit how you convert cm into metres and hence cubic cm into cubic m. The relationship with litres is not so self-evident. In the good (bad?) old days it was a nightmare trying to remember relevant conversion factors if you were working in ergs or dynes or foot-poundals ... Under the same international conventions, the word 'molar' means 'divided by the amount (in moles)' molar mass is measured in kg per mole. All of this means that you can't have the word molar by itself. So you can't have a 2 molar solution. There is a further point, from a teaching point of view, that 2M means a concentration, 2 mol dm-³ and by just writing it as '2M' doesn't actually help the students very much. (In advanced work, people havesometimes referred to a 2 molal solution. Does that mean anything to most technicians?
'M' was a convenient abbreviation but it is not self-evident what it means. Doubtless, busy technicians will continue to use the abbreviation but it shouldn't be used in exams so it's probably not a good idea to use it for pupils who are preparing for exams.
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