Year 10 mocks Chemistry

Find the relative formula mass of MgCl₂ 

First, find the A_r of the elements in the compound - Mg =24 Cl = 35.5

Then add up the relative atomic masses;

24+(35.5X2) = 95 So M_r of Mg Cl₂ is 95

% mass of an element in a compound

(A_r X number of atoms of that element / M_r of the compound)  X100

One mole is 6X10²³. One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass  (A_r or M_r) of  that substance. For example, 

Carbon has an A_r of 12. So one mole of carbon weighs exactly 12g.

Carbon dioxide, CO₂ has an M_r OF 44. So one mole of CO₂ weighs exactly 44g.

Number of moles = mass in g (of an element or compound) / M_r (of the element or compound)

One mole of any gas occupies 24dm³ at 20^oC.

Volume of gas =( mass of gas / M_r of gas) X 24

Volume of 319.5g of Chlorine = 319.5/71 X24 = 108dm³

Concentration

Mass of solute (in grams) / volume of solvent (in dm³) 

Number of moles of solute (mol) / volume of solvent (dm³)

Atom economy

Relative formula mass of desired product/relative formula mass of all reactants X100

Percentage yield

Mass of product actually made / maximum theoretical mass of product X 100