Formula mass
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What is the relative atomic mass? (Ar) Where is it on a periodic table element card?
A way of saying how heavy an atom is compared with the mass of carbon -12. It is just the same as the mass number, the number on the top of the two next to the symbol. (the bigger number)
What is the relative formula mass? (Mr)
All of the relative atomic masses in a molecule added together
What is one mole of a substance = to?
its Mr in grams
What is the formula for Mass, Formula mass and moles?
Number of moles = Mass (g of element or compound) ————————————————– Mr (of element or compound formula mass) Moles = mass over Mr
How do you calculate the %mass of an element in a compound? (formula?)
Percentage mass of an element in a compound = Ar x no. of atoms of the element (O2= 16x2= 32) ———————————————————————- Mr (of the whole compound) (H2O= 18) X100 Ar divided by Mr X100
What are the steps for finding the empirical formula? (4)
• List all the elements in the compound • Underneath, write their experimental masses or percentages • Divide each mass or percentage by the Ar of that element • Turn those numbers into a ratio by dividing them by a common factor, so they are low, e.g (Fe=8, O=12 so make that Fe=2, O=3 by dividing by 4) • With the ratio in its simplest form you have the empirical formula, so with Fe=2, O=3, the empirical formula is Fe2O3 (little 2s and 3s)
How do you find out how much of a reactant was in the experiment? (When you know the final weight of one of the products) Steps… (3)
• Write out the balanced equation • Work out the Mr, just for the sections you want • Divide to get one gram, then multiply to get all the grams…. 48g of Mg …..reacts to give ……80g of MgO (divide to get 1g of Mg, /by 48) 1g of Mg ….. recats to get …. 80/48 of MgO (if you want 60g of Mg to react you x by 60) 60g of Mg ….. recats to get ….. 100g of MgO