Formula_5.4_18.3_5.2_5.3

Question 1

What formula do you use to calculate the Percentage Composition, by mass of an element?

Answer 1

% by mass of the Element

= Mass of Element in 1 mole of compound x 100
Mass of 1 mole of compound

Question 2

What are the steps for calculating the Percentage Composition, by mass of an element?

Answer 2

Step 1: Find the molar mass of the compound (Use Periodic Table)
Step 2: Find the Total molar mass of the element in one mole of the compound
Step 3: Find the % by mass of the element in the compound

Question 3

Calculate the percentage by mass of nitrogen in ammonuum chloride (NH₄CI)

Answer 3

Step 1: (1 x (N)14.0) + (4 x (H)1.0) + (1 x (CI)35.5
= 14 + 4 + 35.5 = 53.5
Step 2: Mass of N in 1 Mole = 1 x 14 = 14.0 g
Step 3: % of Mass of N in NH₄CI = Mass of N in 1 mol of NH₄CI / Molar mass of NH₄CI x 100
= 14.0 / 53.5 x 100

= 26.2%

Question 4

What are the steps used to calculate the Empirical formula of the compound?

Answer 4

Step 1: Determine the mass (m) of each element in the compound (in g’s)
Step 2: Calculate the amount in mole (n) of each element in the compound using n = m / M
Step 3: Calculate the simplest whol number ratio of moles of each element in the compound = Divide Step 2 by the smallest value number of moles calculated in step 2.
Step 4: Find the simplest whole number ratio
Step 5: Write the Empirical Formula

Question 5

Determine the empirical formula of the compound with the following compositions.
42.9% carbon and 57.1% Oxygen

Answer 5

Step 1: m(C) = 42.9g

m(O) = 57.1g

Step 2: n( C) = 42.9 / 12.0 = 3.575

n(O) = 57.1 / 16.0 = 3.56875

Step 3: = 3.575/3.56875 = 1.0018 = 1.0

= 3.56875 / 3.56875 = 1.0

Step 4: 1 : 1

Step 5: CO

Question 6

What is the Molecular Formula?

Answer 6

of empirical formula units in a molecule =

molar mass of the of compound
Mass of 1 empirical formula unit

Question 7

What are the steps for calculating the molecular formula?

Answer 7

Step 1: Determine the empirical formula of the compound
Step 2: Determine the molar mass of one unit of the empirical formula
Step 3: Determine the number of empirical formula units in the molecular formula.
Step 4: Write the molecular formula

Question 8

Determine the molecular formula of the following compounds:
Empirical Formula-CH₂O and Relative Molecular Mass = 90

Answer 8

Step 1: Molar mass of a CH₂O Unit = 12 + (1x2) + 16 = 30.0 g/mol^-1
Step 2: Number of CH₂O Units = 90 / 30 = 3
Step 3: Molecular formula = 3 x CH₂O
Step 4: = C₃H₆O₃

Question 9

What are the steps involved in solving Stoichiometry problems involving excess reactants?

Answer 9

Step 1: Calculate the number of moles of each reactant
Step 2: Determine which reactant is in excess and therefore which is the limiting reactant
Step 3: Use the amount of limiting reactant to work out the amount of product formed.

Question 10

Sodium metal can react with chlorine gas to form sodium chloride. 25.0g of sodium is reacted with 50.0g of chlorine gas.
How much sodium chloride (in grams) is formed? (Teacher Rounded M down from periodic table)

Equation is 2Na(s) + Cl2(q) —> 2NaCl(s)

Answer 10

Step 1: n(Na) = 25 / 23 = 1.087 n(Cl₂) = 50/(2x35.5 ( Teacher Round to 35)) = 0.714

Step 2: I have 1.087 mol (Na) and 2 / 1 * 0.714 = 1.428 mol

Step 3: Na is the limiting reactant.

Step 4: n (NaCl) = n (Na) = 1.087

Step 5: m (NaCl) = 1.087 x 58 (rounded 23 + 35.5) = 63.406 or approx 64.0g

Question 11

What is the formula to calculate the number of Molecules?

Answer 11

n = N / N_A and also equates to N = n * N_A

Question 12

Calculate the number of molecules in 1.6 moles of carbon disoxide (CO2)

Answer 12

Step 1: List the data given in the question
N(CO₂) = ?

n(CO₂) = 3.5 mol

NA = 6.02 * 1023
Step 2: Rearange the formula to make the unknown the subject N(CO₂) = n x N_A
Step 3: Substitute in the data and solve
N(CO₂) = n x N_A
= 3.5 x 6.02 * 10²³
= 2.1 * 10²³

Question 13

What is the formula to calculate the mass of a substance?

Answer 13

m = n x M or n = m / M

where:
n = amount of substance in mol
m = the mass of the substance in grams
M = it’s molar mass in g mol^-1

Question 14

Calculate the mass of 4.68 mol of sodium carbonate (Na2CO3)

Answer 14

Step 1: List the data given in the question
m(Na₂CO₃) = ?g

n(Na₂CO₃) = 4.68 mol

M(Na₂CO₃) = (2 * 23) + 12 + (3 * 16) = 106

Step 2: Calculate the mass using n = m / M or m = n x M
m(Na₂CO₃) = 4.68 x 106
= 496.08
= 496g

Question 15

Calculate the number of CO₂ molecules present in 22 g of carbon dioxide

Answer 15

Step 1: List the data given in the question
n(CO₂) = ?

m(CO₂) = 22 g

M(CO₂) = 12 + (2 * 16) = 44.0 g mol^-1

Step 2: Calculate amount in mol of CO₂ using n = m / M
n(CO₂) = 22 / 44 = 0.5 mol

Step 3: Calculate the number of CO₂ molecules using

n = N / N_A or N = n * N_A
n(CO₂) = 0.5 * 6.02 * 10²³
= 3.0 * 10²³