You first have to understand that the mole i a concept. Its generally used in chemistry and physic to measure mass, but it can also technically be used in other situation. It used to measure a quantity of "something". Anything. In science, a quantity of matter i literally synonymous to mas, and that why it used in place of a mas unity sometimes. In our situation, mol/dm^3 is used for concentration because concentration is defined a being a measure of mas divided by volume (assuming dm3 is already understood a being a measure of volume). A mole i defined a being equal to 6.022x10^23 unitie. From now on, we will refer to this ugly number a "Na" (dont get mixed up with sodium).
So, to recap, a mole is a given number of thing that can be used instead of a mass unity. Let get to examples. So lets say you have 10g of salt (NaCl) in 1l(or 1dm^3, 1l being equal to 1dm^3) of water. The cool thing about Na i that it can convert from Atomic Mass Unitie(or Molecular Mas Unities) to gram. Whats even cooler i that because thi number is so perfect, you dont need to make any calculation to convert. So in 1 mol of salt, you will have 58(23 for the sodium +35 for the chlorine) grams. Now since we know that there i 58g in 1 mol, how much mole will there be in 10g? I hope you answered around 0,172... if you didnt, heres the explanation. There are 58g of salt in 1 mole. You have only 10g. You have obviously les than 1 mole. In fact, you have 10 58ths of a mole. 10 divided by 58 equals 0.172413793. To solve your other problem, dont hesitate on adding me a a contact so you can ask question directly.
