# Relative Masses Ar and Mr Relative Atomic mass

• Slides: 19

Relative Masses - Ar and Mr Relative Atomic mass Ar is the mass of the atom when compared to Carbon. The mass of Carbon is defined as 12. 00. This value can be found on the periodic table Relative Molar mass Mr is the combined mass of all the atoms in a molecule. It is found by adding the masses of all the atoms in a molecule.

Calculating Molecular mass When calculating molecular mass we always set it out in a proscribed way. Eg. Mass of glucose Mr(C 6 H 12 O 6) = [6 Ar(C)] + [12 Ar(H)] +[6 Ar(O)] = (6 x 12) + (12 x 1) + (6 x 16) = 180

Calculate the molecular mass of the following • • • Mg. Cl 2 H 2 SO 4 N 2 C 2 H 4 Al(OH)3 Na 2 CO 3

The Mole (N) • A mole is the number of carbon atoms in 12. 000 g of carbon-12 • 1 g of Hydrogen and 12 g of Carbon contain the same number of atoms. • For each element in Ar grams of that element there is one mole of atoms • One Mole = 6. 02 x 1023 • It is also called Avogadro’s number

Molar Mass (M) • The mass of one mole of something is called its molar mass, (M) • Eg. Ar(S) = 32 M(S) = 32 g mol-1

Molar calculations To figure out the number of moles in a substance we use the formula or It can be rearranged to give the formulas

Example What is the mass of 5 moles of Carbon dioxide? M(CO 2) = 44 g mol-1 n(CO 2) = 5 mol M(CO 2) = 44 g mol-1 m = n. M = 5 mol x 44 g mol-1 = 220 g m(CO 2) = ?

Practise calculations Calculate the mass of 1 mol O 2 2 mol SO 2 ½ mol CO 2 How many moles are there in 190 g of Mg. Cl 2 0. 28 g of Fe 56 g N 2

Experiment Read page 81 Visit each of the stations around the room, and while at each station fill in the row in the data table on pg 83

A mole of a molecule can contain more than one mole of each of the constituent atoms i. e. 1 mol of H 2 O contains 1 mol of Oxygen and 2 mol of Hydrogen Eg. Moles of Cl- in 95 g Mg. Cl 2 M(Mg. Cl 2) = 95 g mol-1 m(Mg. Cl 2) = 95 g n(Mg. Cl 2) = m/M = 95 g/95 g mol-1 = 1 mol Mg. Cl 2 in 95 g Mg. Cl 2 n(Cl-) in Mg. Cl 2 = 2 x n(Mg. Cl 2) = 2 x 1 mol = 2 mol

How many moles of Al 3+ in 10. 2 g Al 2 O 3 How many moles of O in 98 g of H 2 SO 4 How many moles of N in 56 g of N 2

Using the mole A balanced equation tells us the number of moles of each reactant and product. Eg 2 H 2(g) +O 2(g) H 2 O(l) The number of moles of H 2 is twice that of O 2 The number of moles of O 2 is half that of H 2 The number of moles of H 2 is equal to that of H 2 O

Calculating mole ratios Decide which is the known and which is the unknown substance Write the mole ratio of unknown to known with unknown on top using the coefficients in the balanced equation Put the values into the expression to calculate the volume of the unknown substance

Using Mole Ratios: example 2 C 2 H 6(g) +7 O 2(g) 4 CO 2(g) +6 H 20(l) How many moles of oxygen are required to burn 5 moles of ethane? n(C 2 H 6)= 5 mol n(O 2)= ?

Mole calculations Where the number of moles of one reagent is known the number of the other can be calculated. The basic method for solving mole calculations is 1. Write a balanced equation for the reaction 2. Calculate the number of moles of the known reagent 3. Use the balanced equation to calculate the number of moles of the unknown reagent 4. Calculate the mass of the unknown reagent

Practise questions Pb(NO 3)2(aq) +2 Na. Br(aq) Pb. Br 2(s) + 2 Na. NO 3(aq) How many moles of Pb(NO 3)2 react with 0. 060 moles of Na. Br? How many moles of Pb. Br 2 would be formed from 2. 8 x 10 -3 mol of Pb 2+(aq)? How many moles of Na. Br would have reacted to make 0. 4 mol of Pb. Br 2? How many moles of Pb(NO 3)2 would have reacted to make 0. 5 mol of Na. NO 3?

Al(OH)3(s)+ 3 HCl Al. Cl 3(aq) +3 H 2 O(l) How many moles of HCl are required to react with 3. 6 moles of Al(OH)3? How many moles of Al(OH)3 will react with 0. 042 mol of HCl?

Mg(s) + 2 HNO 3(aq) Mg(NO 3)2(aq) + H 2(g) 0. 25 mol of Mg is reacted with excess nitric acid How many moles of H 2(g) are produced? If one mole of H 2 gas has a volume of 24 L, what volume of H 2 gas is produced? How many moles of nitric acid reacted? If 2 moles of nitric acid were used, how much of the unreacted acid remains?

40. 0 g of Cu. CO 3 were heated and decomposed. Ar(Cu) = 63. 5; Ar(C) = 12. 0; Ar(O)=16. 0 Write the balanced equation for the decomposition of Cu. CO 3 to make Cu. O? Calculate the molar mass of Cu. CO 3 ? Calculate the amount, in moles, of Cu. CO 3 used? Calculate the molar mass of Cu. O? What is the amount, in moles, of Cu. O formed? Calculate the mass of Cu. O formed?