Chapter 8

The number of particles in a mole is called as Avogadro’s constant or number. This unit called the mole, is defined as the number of atoms in exactly 12 g of carbon-12.

One mole represents 6.022х10²³ particles. The particles can be atoms in elements, molecules in molecular compounds, or ions in ionic compounds.

Avogadro’s constant can be used to convert an amount in moles to an equivalent number of atoms. Example 2.66 mol of an element can be converted to number of atoms by multiplying the number of moles with Avogadro’s number.

Determine the number of atoms present in 2.5 mol of silicon.

Convert 3.01х10²³ atoms of silicon to moles of silicon.

How many atoms are present in 3.7 mol of sodium?

How many moles of silver is 2.888х1015 atoms?

Sometimes pure metals like copper may be a mixture of two isotopes. In any sample of copper, 69.17% of the atoms are copper-63 and 30.83% of the atoms are copper-65.

Copper has a relative atomic mass of 63.54 amu. In nature, copper consists of a mixture of two isotopes,copper-63 and copper-65.The abundance of copper-63 is 69.17% and that of copper-65 atom is 30.83%. The relative atomic mass of copper is a weighted average of these two isotopes.

The mass of a copper-63 atom is 62.94 amu and that of copper-65 atom is 64.93 amu. Using this data and the abundance of these two isotopes, determine the average atomic mass of native copper. The abundance of copper-63 is 69.17%. The abundance of copper-65 is 30.83%.

Page 279 1,2,3.

Molar mass is the mass in grams that is equal to the sum of all the atomic masses of the component atoms of a substance. The molar mass of an element is numerically equal to the element’s atomic mass and has a unit of g/mol

To convert amount in moles to mass you multiply number of moles with the molar mass of that element.

To convert mass to amount in moles you divide the given grams of the element with the molar mass of the element.

Determine the number of moles represented by 237 g of copper.

Determine the mass in grams of 3.50 mol of copper.

To find the mass of an atom you divide the molar mass of an atom with Avogadro’s number.

Find the mass of a single silicon atom.

Page 283 Q.13,15,16 do all the problems under

these.

Formula reveal composition: A formula tells us about the elements that are present in a compound. Example in KBr compound is made up of the elements potassium and bromine.

Also tells us about the ratio in which these two elements are present.Potassium bromide is made up of ions in the ratio of one potassium cation, K+ for each bromide anion, Br— .

The meaning of a formula does not change when polyatomic ions are involved. Potassium nitrate has the formula KNO₃. Similar to KBr this compound also has a ratio of 1K+ cation and I NO₃− anion.

What is a formula unit of an ionic compound?

Formula unit is the unit that represents the simplest ratio of cations to anions. A formula unit of NaCl consists of one Na+ and one Cl-

One mole of an ionic compound consists of 6.022х10²³ of these formula units. One mole of a molecular compound, like H₂O consists of 6.022х10²³ molecules.

The molar mass of an ionic compound or molecular compound is the sum of the masses of all the atoms in the formula expressed in grams.

The molar mass of water will be calculated in the following way;

Page 288 Q.1 do # a,c, d, j and k Q.2. b,d,f and h.

What is percentage composition? The percentage by mass of each

element in a compound is the percentage composition. Percentage composition data allows us to calculate the simplest ratio among the elements of a compound. A formula that represents this simplest ratio is called an empirical formula.

One of the substances in a new alkaline battery is composed of 63.0% manganese and 37.0% oxygen by mass. Determine the empirical formula of the compound in the battery?

Chemical analysis of a clear liquid shows that it is 60.0% C, 13.4% H, and 26.0% O.Calculate the empirical formula of this substance.

Molecular formulas are multiples of empirical formulas.

n(empirical formula)=molecular formula Molecular formula tells us about the

actual number of atoms present in a compound

One C-H empirical formula unit Six C-H empirical units bonded together to form a molecule of

benzene with molecular formula C

66

The empirical formula of a compound is found to be P₂O5. The molar mass of the compound is 284 g/mol. Determine the molecular formula of the compound.

Determine the percentage of water by mass in sodium carbonate decahydrate, Na2CO3.10H2O

Page 297 Q.7 c and d Q.8 a,b,e Q.12 all Page 305 Test prep all