# Do Now - What is the molar mass?

Do Now - What is the molar mass? 1.MgO 2.CO2 3.CuCO3 4.CuO 5.HNO3 6. NH3 7. NaOH

8. (NH4)2SO4 9. H2SO4 What is the molar mass? 1.MgO40.3 2.CO2 44.0 3.CuCO3 123.5 4.CuO 79.5 5.HNO3 63.0

6. NH3 17.0 7. NaOH 40.0 8. (NH4)2SO4 132.1 9. H2SO4 98.1 Mole = mass Converting moles Mr

Avogadros number = no of atoms or molecules in 1 mole Using Avogadro Number Q1. 0.450 moles of Fe contains how many atoms? Q2. 0.200 mole of H2O contains how many molecules? Using Avogadro Number: read the question carefully!!

Q3. 0.450 g of Fe contains how many atoms? Q4. 0.200 g of H2O contains how many molecules? More calculations: 1) Calculate the number of molecules in 1.058 mol of H2O 2) Calculate the number of atoms in 0.750 mol of Fe 3) Calculate the number of molecules in 1.058 g of H2O 4) Calculate the number of atoms in 0.750 g of Fe

5) Which contains more molecules: 10.0 g of O2 or 50.0 grams of iodine, I2? Multiple choice 1 Which of the following pure iron samples contains the largest number of atoms? a. 6.70 g b. 0.110 mole c. 7.83 x 1022 atoms Learning Objectives

Understand how to do calculations using mass concentration, molar concentration and using concentration and mass Calculate solution concentrations, in moldm-3 and gdm-3 Think back to KS3 ... Write down what is meant by the following words: 1. Solute 2. Solvent 3. Solution

The definitions and calculations are the SAME at A Level!! Think back to KS3 ... Write down what is meant by the following words: 1. Solute substance that is dissolved 2. Solvent substance that dissolves a solute 3. Solution a solute dissolved in a solvent

The definitions and calculations are the SAME at A Level!! Calculations using mass concentration Mass concentration in gdm-3 = mass of solute in g volume of solution in dm3

Examples 1. 200cm3 of a solution contains 5.68g of sodium bromide. What is the mass concentration? 2. The concentration of a solution is 15.7g dm-3. Was mass of solute is there in 750cm3 of solution? 3. A chemistry uses 280g of a solute to make a solution of concentration 28.4g dm-3. What volume of solution does he make? Units of Concentration

The concentration of a solution tells us how much solute is present in a known volume of solution. Concentrations of solutions are measured in the unit moles per cubic decimetre, mol dm-3. We often use the symbol M for Molar. This is the old-fashioned term for mol dm-3 1 mol dm-3 means there is 1 mole of solute per cubic decimetre of solution; 2 mol dm-3 means there are 2 moles of solute per cubic decimetre of solution and so on and so on ...

Converting Units 1 decimetre = 10cm 1 one cubic decimetre, 1 dm3, is 10cm x 10cm x 10cm = 1000cm3 This is the same as 1 litre. 1dm3=1L To conver t cm 3 to d 3

m (or L) Divide cm 3 by 1000 How to find the concentration in mol dm-3 If the units of concentration are mol dm-3, what 2 pieces of information would you need to calculate concentration? Moles and volume What information do you need to work out moles? Mass and Mr

Can you come up with an equation that links concentration, moles and volume? Concentration = Moles Volume c= n V Units: Concentration in mol dm-3 Volume in dm3

Moles in mol How to find the concentration in mol dm-3 Example: 1.17g of sodium chloride was dissolved in water to make 500cm3 of solution. Step 1: Work out the number of moles of NaCl Mol= Mass Mr Mol= 1.17 = 0.0200 mol (3sf) 58.5

Step 2: Use the number of moles and the volume to work out c Concentration = moles = 0.0200 = 0.0400 mol dm-3 Volume (500/1000) Rearranging the Equationc = n Rearrange the equation to find: Number of moles n=cxV Volume

V=n c V Number of Moles in a Given Volume of Solution You often have to work out how many moles are present in a particular volume of a solution of known concentration.

Use the formula: n=cxV But remember that V is usually given in cm3, so you must convert them to dm3. Example How many moles are present in 25.0cm3 of a solution of concentration 0.10mol dm-3?

Number of moles in solution = c x V = 0.10 x (25/1000) = 0.0025 mol Worked Examples: 1 What is the concentration of 0.25 dm3 of a solution containing 0.50 mol of sodium hydroxide? c=n V c = 0.50 0.25

= 2.0 mol dm-3 Worked Examples: 2 0.050 mol of sodium carbonate was dissolved in water and added to a 250cm 3 volumetric flask. The solution was made up to the mark with water. What is its concentration? c=n V c = 0. 05 (250/1000)

= 0.200 mol dm-3 Worked Examples: 3 What volume of 0.10 M hydrochloric acid contains 2.5x10-3 mol? V=n c V = 2.5 x 10-3 0.10 = 2.5x10-2 dm3 (25cm3)

Worked Examples: 4 What mass of sodium hydroxide is needed to make 250cm3 of a 0.500 M solution? Step 1: n=cxV n =0.500 x (250/1000) = 0.125 mol Step 2: mass = Mr x n = 40.0 x 0.125 = 5.00 g

Practise Questions ... 1. Calculate the concentrations of the following: a. 1.0 mol of solute in 2.0 dm3 of solution b. 2.0 mol of solute in 250 cm3 of solution c. 2.0 g of sodium hydroxide in 25 cm3 of solution EXTRA QUESTIONS AVAILABLE FROM ME!!

2. How many moles of solute are dissolved in: a. 0.5 dm3 of a 2.5 mol dm-3 solution b. 250 cm3 of a 0.05 M solution? 3. What mass of anhydrous sodium carbonate Na2CO3, is needed to make 250 cm3 of a 0.10 M solution? Practise Questions ... 1. Calculate the concentrations of the following: a. 1.0 mol of solute in 2.0 dm3 of solution 0.50 mol dm-3 b. 2.0 mol of solute in 250 cm3 of solution 8.0 mol dm-3

c. 2.0 g of sodium hydroxide in 25 cm3 of solution 2.0 mol dm-3 2. How many moles of solute are dissolved in: a. 0.5 dm3 of a 2.5 mol dm-3 solution 1.25 mol b. 250 cm3 of a 0.05 M solution? 0.00125 mol 3. What mass of anhydrous sodium carbonate Na2CO3, is needed to make 250 cm3 of a 0.10 M solution? 2.65 g Challenge a) An excess of magnesium is added to 100cm3 of 1.50moldm-3 dilute hydrochloric acid. What mass of hydrogen is formed?

b) A mass of 47.8g of magnesium carbonate reacts with 2.50moldm-3 of hydrochloric acid. What volume of acid is needed? Multiple choice 2 Which of the following contains the most chloride ions? A 10 cm3 of 3.30 102 mol dm3 aluminium chloride solution B 20 cm3 of 5.00 102 mol dm3 calcium chloride solution C 30 cm3 of 3.30 102 mol dm3 hydrochloric acid D 40 cm3 of 2.50 102 mol dm3 sodium chloride solution

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