Please give me a crash course on mols? (1 Viewer)

[Kingportable]

Hi guys I joined a chemistry after they finished with the mols topic and i'm really stuck on use the formulas and are quite helpless with the questions, lol I was shocked when someone said that I had to convert everything to liters or somethingwhen using or figuring out for mass.

So you can tell I'm clueless, lol so if anyone is "kind" and "GENIUS" enough plox lol give me a crash course on at least the basics of mols questions

 

[John Rips]

Basically most questions require you to convert a mass to no of moles of a substance. Work out the Molar Mass by adding the elemental atomic masses found in the periodic table and apply the formula n = m/M where n = no of moles, m = mass of substance you have and M = molar mass. For ex: lets say you reacted 20g of Ca(OH)2 with excess HCl to form CaCl2 and H2O and you are asked, what mass of CaCl2 is formed? 1st. Always balance the formula, the coefficients give you the molar ratio. Here is 1 of Ca(OH)2 + 2 of HCl gives 1 of CaCl2 + 2 of water. so Molar ratio is 1:2 = 1:2 and the ratio of Ca(OH)2 used to CaCl2 formed is 1:1. Molar mass of the compound is Ca = 40 added to 2 of (O = 16 + H = 1) = 74 g/mol. so no of moles of Ca(OH)2 used is 20/74 = 0.27 mol, so you will produce 0.27 mol of CaCl2. Go back to periodic table and formula to work out the mass. Molar Mass CaCl2 = Ca (40) + 2 of Cl (35.5) = 111 g/mol. mass = n x M = 0.27 x 111 = 30g of CaCl2 produced. Most other questions rely on same procedure. If its gases, use the gas rules which are n = vol of gas / Molar gas volume at that temperature (usually 22.4 L at STP). Hope that sorta helps. Would take ages to tell you it all. John