S1-L4: Chemical equations, The Mole & Molarity

Question 1

Define a “mole”

Answer 1

• one mole of any molecule contains Avogadro’s n. of molecules
• Is SI unit of amount of substance
• ->1 mole of any element contains same n. of atoms (Avogadro constant)

Question 2

What is one mole equal to?

Answer 2

-6.022137 x 10^23

Question 3

Outline what Avogadro’s number is

Answer 3

• (Na) defined as n. of atoms found in 12 g of C 12

- one mole of any molecule contains Avogadro’s n. of molecules

Question 4

What is the mass of one mole of any substance?

Answer 4

-its relative atomic mass (elements) OR relative molecular mass (compound) in grams

Question 5

Describe “molar mass” (figure 1)

Answer 5

• mass of one mole of substance
• ->unit is g/mol
• useful to calculate mass relationship in chemical reactions

Question 6

What is “molecular mass”? Use the example of SO2

Answer 6

• 1S –> 32.07
• 2O –> (2x16.00)
• SO2 –> 64.07 so 1 molecule of SO2 is 64.07 amu–> 64.07

Question 7

Outline what one mole of any gas occupies

Answer 7

-same volume at standard temp AND pressure

Question 8

State the ideal gas law and what the different parts mean

Answer 8

-pV=nRT
-n= n. of moles/ R= a constant/ V= 22.4 L/ T= 273 K/
P= 100 kPa (IUPAC definition)

Question 9

Example: What is the n. of moles of a molecule of 10g of NaCl?

Answer 9

• 10/(35.5 + 23) = 0.171

Question 10

Define “molarity”

Answer 10

-solution conc expressed as moles of solute dissolved in 1 litre of solution

Question 11

What is the molarity formula?

Answer 11

-molarity= n. of moles/ volume in litres

Question 12

Example: What mass of glucose needed to prepare 500 mL of 1 M solution?

Answer 12

• 1 mol/L= n. of moles (0.5 L) n=0.5
• Glucose (C6H12O6): (6x12.01) + (12 x 1.008) + (6x16)= 180.16 g/mol
• ->0.5 moles weight 0.5 x 180.16 g/mol = 90.08 g

Question 13

State what “1mL” and “1 mg” is

Answer 13

• 1 mL= 1/1000 L

- 1 mg = 1/1000 g

Question 14

What do chemical reactions involve?

Answer 14

• compound identity changes
• energy changes
• breaking AND making chemical bonds
• rearrangement of valence e-‘s

Question 15

How do chemical equations “describe” chemical reactions?

Answer 15

• symbols represent elements
• formulae describe composition of compounds
• equations show changes in compounds

Question 16

Define a “chemical equation”

Answer 16

-symbolic representation of chemical reaction

Question 17

Finish off the following chemical reaction equations:

1- 2Na + Cl2…
2- CH4 + 2O2…
3- 4Al + 3O2…

Answer 17

1 –>2NaCl
2 –> CO2 + 2H2O
3 –> 2Al2O3

Question 18

What is the general structure of a chemical equation?

Answer 18

-Reactants–> Products

Question 19

Outline what coefficients are in terms of chemical reactions

Answer 19

• number which may be included in reaction equation before formula
• if no number then “1” known to be in front of formula

Question 20

What do coefficients represent in the chemical reactions?

Answer 20

-the n. of molecules of that compound/ atom needed in reaction

Question 21

Why are coefficients important in balancing equations?

Answer 21

-can change coefficient to achieve mass balance BUT subscripts can never be changed to balance equation

Question 22

Outline what the following equation means in terms of states and atoms/molecules reacting:

Zn(s) + 2HCl (aq)–> ZnCl2 (aq) + H2 (g)

Answer 22

-1 atom (65g) of solid Zn reacts with 2 molecules (73g) of aqueous HCl acid to form 1 molecules (136g) of aqueous ZnCl2 and 1 mole (2g) of gaseous H

Question 23

What is the meaning of the “yield of reaction”?

Answer 23

-the % of predicted yield actually obtained

Question 24

Example: Using the same reaction of Zinc reacting with Hydrochloric acid what would the yield of the reaction be if you isolate 115 g of ZnCl2 from the reaction?

Answer 24

-(115/136) x100 = 84.6%

Question 25

Why is balancing equations important?

Answer 25

• nothing comes from nothing
• can’t create/lose matter from closed system
• ->so must balance equation for each side to represent equal quantity of each element

Question 26

Example reaction: Na reacting with Cl to form NaCl

What would the reaction equation be?

Answer 26

• Na + Cl2 –> NaCl

- -> 2Na + Cl2 –> 2NaCl

Question 27

Reaction: 2C2H6 + 7O2 –> 4CO2 + 6H2O
Na2SO4/ CuCl2

Outline the first step to balancing chemical equations

(figure 2)

Answer 27

-write correct formula(e) for reactants left side AND correct formula(e) for products right side of equation

• C2H6 + O2 –> CO2 + H2O
• Reminder: write symbols for elements AND ions: Na/ SO4/ Cu/ Cl
• write valencies above AND right of symbols

–>Na^1SO4^2 Cu^2^Cl^1
–>reverse valencies and write numbers below AND to right:
Na2SO4 AND CuCl2

Question 28

What is the second step to balancing equations?-explain

Answer 28

• change coefficients on front of molecular formulae to make n. of atoms of each element same both sides of equation
• ->don’t change subscripts
• 2C2H6 –> 2 molecules of methane AND C4H12–> not exist
• subscripts indicate n. of bonded atoms in molecule
• ->changing subscripts changes compound

Question 29

State step 3 in balancing equations

Answer 29

• balance most complicated molecule first
• ->this is the one which contains many different atoms
• then use smaller molecules to adjust for this major change
• balance elements last

Question 30

What is the final step to balance equations? (figure 3) and state the final balanced equation

Answer 30

• check to make sure have same n. of each atom type both sides of equation
• final equation: 2C2H6 + 7O2 –> 4CO2 + 6H2O

Question 31

Key points to balancing equations

Answer 31

• don’t separate complex ions which not changed in reaction such as
• -> SO4 2-/ NH4+/ CO3 2-/ HCO2/ NO3-
• ->can often be considered as unit- not individual elements
• sometimes (like thermochemistry) showing state of each component of reaction important

Question 32

Summary of lecture

Answer 32

• mole–>molar mass/ molecular mass/ molarity of solution
• chemical reactions
• chemical equations–> writing chemical equations/ balancing equations