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Chemistry Calculations

Chemistry Calculations. Learning Intentions. How can we work out the formula mass of a chemical? What is a mole? How can we use formula mass and moles to work out grams of a chemical?. Part 1: Formula Mass. Each element has a Mass Number e.g. Na = 23, O = 16. Task:

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Chemistry Calculations

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  1. Chemistry Calculations

  2. Learning Intentions • How can we work out the formula mass of a chemical? • What is a mole? • How can we use formula mass and moles to work out grams of a chemical?

  3. Part 1: Formula Mass Each element has a Mass Number e.g. Na = 23, O = 16 Task: Use your data book to find the mass numbers for: Ca, Cl, S, H, N, C, Ar

  4. Formula Mass If we add up all the mass numbers present in a chemical we get the Formula Mass number Example: H2O = 2 X Hydrogen + 1 X Oxygen Mass Numbers = 2 X 1 + 1 X 16 = A formula mass of 18

  5. Task: • NaOH • HCl • O2 • HNO3 • H2SO4 • (NH4)3PO4

  6. (NH4)3PO4 Solution: Split into 2 halves 2) PO4 P = 31 X 1 O = 16 X 4 PO4 = 95 1) (NH4)3 N = 14 X 1 H = 1 X 4 NH4 = 18 (NH4)3 = 18 X 3 = 54 54 + 95 = a formula mass of 149

  7. Part 2: The Mole Units of Measurement: We use units of measurement in life to give us an easier way to see large numbers. metre = 100 cm, litre = 1000 ml, kilogram = 1000 g The mole is a unit of measurement In chemistry we use it to tell us how many atoms or molecules there are

  8. The Mole The next slide is Higher Chemistry but will help you understand what a mole is

  9. The Mole Example: If I had 1 mole of grapefruits I would have 602,214,150,000,000,000,000,000 (6.02 X 1023) grapefruits which would be the same size as If I had 1 mole of H2O I would have 6.02 X 1023 molecules of water.

  10. How big is a mole? • A mole of sand grains would cover the United States in approximately one centimetre of sand.  • A human body contains roughly one hundred trillion cells; there are roughly six billion people on Earth; so the total number of human cells on the planet is very close to one mole.  •  If you had exactly one mole of sheets of paper, you could make one million equal stacks from sea level on the earth that would pass the sun.  • If you had a mole of pennies, you could give out enough money to everyone in the world so that they could spend a million dollars every hour, day and night, for the rest of their lives.

  11. The Mole If I could weigh 6.02 X 1023molecules of water it would weigh 18g Q) What is the formula mass of water? 18 Q) What is the relationship between the formula mass and 1 mole of a chemical? 1 mole of a chemical is equal to the Formula Mass in grams

  12. Example I have 40g of NaOH The Formula Mass (FM) = 40 Therefore I have 1 mole NaOH FM = 40 1 mole = 40g 0.5 moles = 20g 0.25 moles = 10g etc

  13. Part 3: Mole Calculations We can use moles and formula mass to work out mass of chemicals needed/used M = n X FM n = M/FM FM = M/n (n)

  14. Example If I have 9g of water, how many moles do I have? n = M/FM n = 9g/18 n = 0.5 What is the mass of 0.25 moles N2? (n) M = n X FM M = 0.25 X 28 M = 7g

  15. Task: 1) If I have 9g water how many moles do I have? 2) If I have 5g NaOH how many moles do I have? 3) If I have 0.5 moles O2 how many grams do I have?

  16. Chemistry Calculations 2

  17. Last Lesson • How can we work out the formula mass of a chemical? • What is a mole? • How can we use formula mass and moles to work out grams of a chemical?

  18. Mole Calculations M = n X FM n = M/FM FM = M/n (n)

  19. Task: • What is the formula mass of Mg(NO3)2 • What is the formula mass of potassium carbonate? • Calculate the number of moles of 5.6g of CO • Calculate the number of moles of 8.2g calcium nitrate • Calculate the mass of 0.2 moles of magnesium oxide • Calculate the mass of 0.25 mole of NaCl

  20. Learning Intentions • What is concentration? • What are the units for concentration? • How do we calculate moles in solutions?

  21. Part 1: Concentration This bottle of acid is labelled “conc.” What does that mean? Today we learn how to work out how “strong” a solution is

  22. Concentration “ the amount of solute (the thing that gets dissolved) dissolved in a volume of solvent (the thing that does the dissolving)”

  23. Part 2: Units for Concentration Moles per litre written as mol/L or mol l-1 Note: on bottles of solutions we may see concentration labelled as “M” (e.g. 1M)

  24. Part 3: Mole Calculations To work out the exact concentration of a solution we use this triangle: Number of Moles Volume (L) Concentration (mol/L)

  25. Example If I want to make 0.5Lof KOH (potassium hydroxide) that has a concentration of 1mol/L how many moles KOH do I need to dissolve? n = C X V n = 1 mol/L X 0.5L n = 0.5 moles KOH needed

  26. Example 2 What is the concentration of a solution of HCl that contains 0.1 moles of HCl dissolved in 250cm3 Convert to litres! C = n/V C = 0.1/0.25L C = 0.004 mol/L

  27. Using both triangles together For some calculations we need to use both triangles together

  28. Calculate the number of grams of NaOH needed to make 50cm3 of NaOH with a concentration of 2 mol/L First Triangle n = ? C = 2 mol/L V = 0.05L n = C X V n = 2mol/L X 0.05L n = 0.1 mole NaOH Second Triangle M = ? n = 0.1 moles FM = 40. M = n X FM M = 0.1 moles X 40 M = 4g 4g NaOH to make 50cm3 solution with a concentration of 2 mol/L

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