Question

how is the measurement unit,the mole,different from the measurement unit,in kilograms?

Answer 1

Saying you have 'one mole' of a substance is like saying you have 'a dozen'. If we had 1 kg of apples and 1 kg of oranges, we couldn't necessarily claim that we had exactly the same number of apples and oranges. However, if we were to have one mole of a pure atomic substance, such as a sample of sodium, and another one mole of a sample of atomic magnesium, for example, then we could definitively say that each sample contained 6.022 x 10^23 (known as Avogadro's Number) individual sodium and magnesium atoms respectively. That's not to say the two samples are the same weight; the 1 mole sodium sample would weigh 23 grams (approximately) and the 1 mole magnesium sample 24 grams (approximately). We work these out from the relative atomic masses for Na and Mg respectively, found beside each element on the periodic table.

A 1 mole sample of an atomic substance will be equivalent to having a sample of mass (in grams) equal to the relative atomic mass for an atom (you can check this out for yourself with different elements on the periodic table). SO, YOU CAN ONLY CONVERT BETWEEN MOLES AND MASS, THEY ARE NOT THE SAME THING. As atoms of different elements have different masses, if we have a sample of two elements and we wanted to find out how many atoms were in each, it wouldn't be reliable to simply measure which was heaviest, we need some universal unit which describes how many individual atoms are actually present, and that's where the mole comes in.

The same applies to compounds and molecules, for example water (H2O). If we had one mole of water, then we would have 6.022 x 10^23 individual molecules of water in the sample (and the sample would have a mass of 18 g, equivalent to H2O's relative molecular mass in grams). Like 'a dozen' apples, oranges, pears etc....the mole can be applied to any substance. It can be a very tricky thing to get your head around at first, but the main thing is to differentiate between mass and moles - the amount of something you have is not directly related to its weight. Hope that helps! :)