 # Counting Atoms Chapter 9. MOLE?? Moles of Particles In one mole of a substance, there are 6 x 10 23 particles.

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Counting Atoms Chapter 9

MOLE??

Moles of Particles In one mole of a substance, there are 6 x 10 23 particles

Calculating the number of moles from the number of particles TB P147 1. Calculate the number of moles of particles in a.2.4 x 10 23 atoms of hydrogen, H b.4.2 x 10 24 molecules of carbon dioxide, CO 2 c.3 x 10 22 copper ions, Cu 2+ Number of moles = Number of particles 6 × 10 23

Calculating the number of moles from the number of particles TB P147 2. Calculate the number of particles in a.3 moles of carbon atoms, C b.0.75 moles of ammonia molecules, NH 3 c.25 moles of sodium chloride, NaCl Extension Question! How many atoms are there in 0.75 moles of ammonia molecules? Number of moles = Number of particles 6 × 10 23

Molar Mass The mass of one mole of any substance is called the molar mass. The molar mass is the relative atomic mass of the element in grams. For Substances Consisting of Atoms The molar mass is the relative molecular mass of the substance in grams. For Substances Consisting of Molecules The molar mass is the relative formula mass of the substance in grams. For Substances Consisting of Ions

Calculating the number of moles from the molar mass Number of moles = Mass (in g) Molar mass (in g) TB P150 1. (a) How many moles of atoms are contained in 32 g of oxygen atoms? 4. (c) Calculate the mass of 4 mol of oxygen molecules, O 2 ?

Different Kinds of Formulae The molecular formula of a compound shows the actual number and kinds of atoms present. The empirical formula of a compound shows the simplest whole number ratio of the atoms present. The structural formula of a compound shows how the atoms are joined in the molecules.

Calculating the Empirical Formula The empirical formula can be calculated from the masses of elements that combine together. Follow these steps…* It is important to present the answers in a table! Element Mass Molar mass Number of moles Simplest ratio The empirical formula can also be found from its percentage composition. We assume that 100 g of the compound was analysed.

Let’s try! TB P154 1. (a) Find the empirical formula of a compound consisting of 3.5 g of nitrogen combined with 8.0 g of oxygen. 2. (a) Find the empirical formula of a compound with composition 75% carbon and 25% hydrogen by mass. Element Mass Molar mass Number of moles Simplest ratio

From Empirical Formula to Molecular Formula You will need the relative molecular mass of the compound! TB P154 4. The empirical formula of a compound is COH 3. Its relative molecular mass is 62. Find the molecular formula. Let the molecular formula be C n O n H 3n M r of C n O n H 3n = n(12) + n(16) + 3n(1) = 31n 31n = 62 n = 62/31 = 2 Therefore, the molecular formula is C 2 O 2 H 6.

From Empirical Formula to Molecular Formula TB P154 5. The empirical formula of a compound is H 2 CO. Its relative molecular mass is 90. Find the molecular formula. Let the molecular formula be _____________ M r of _____________ = Therefore, the molecular formula is _________.

Molar Volume of GASES Avogadro’s Law Equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules.

Molar volume at r.t.p. it is the volume occupied by one mole of the gas at room temperature, one mole of gas has a volume of 24 dm 3 or 24 000 cm 3 the molar volume is the same for all gases

Calculating the number of moles of a gas from its volume TB P157 1. (b) Calculate the mass of 4.8 dm 3 of carbon dioxide, CO 2 at r.t.p.. 2. (b) Calculate the volume of 2 g of hydrogen, H 2 at r.t.p.. Number of moles of gas = Volume of gas Molar volume of gas

Concentration of Solutions Compare the concentrations of the following solutions. Solution A and C has the same concentration. Solution A has half the concentration of Solution B.

The Idea of Concentration The concentration of a solution tells us the amount of solute in a unit volume of a solution. Two ways to measure concentration: 1. the mass (in grams) of solute in 1 dm 3 of solution g/dm 3 2. the number of moles of solute in 1 dm 3 of solution mol/dm 3

Calculations involving Concentration of Solution Concentration in g/dm 3 = Number of grams of solute Volume of solute (in dm 3 ) Concentration in mol/dm 3 = Number of moles of solute Volume of solute (in dm 3 )

Let’s Try! TB P160 1. Calculate the concentrations of the following solutions, in (i) mol/dm 3, and (ii) g/dm 3. a. 0.4 g of H 2 SO 4 in 2 dm 3 of solution b. 0.5 mol of HCl in 250 cm 3 of solution 5. What mass of potassium hydroxide, KOH, must be dissolved in 500 cm 3 of solution to prepare a 0.4 mol/dm 3 solution?

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