Quantitative Chemistry

Question 1

What is the number used to represent moles?

Answer 1

6.02x10^23

Question 2

What does the empirical formula represent?

Answer 2

This shows us the simplest whole number ratio of the atoms present in a compound

Question 3

What does the molecular formula represent

Answer 3

The molecular formula shows the actual number of atoms of each element present in a molecular or formula unit of a compound

Question 4

What is the formula involving mass, moles and Mr?

Answer 4

Moles = Mass/Mr

Question 5

Describe the process of calculating the empirical formula

Answer 5

Masses(given in question)
Moles Remember, moles = mass/mr so its Masses/mr
Whichever one of the moles is smaller, divide them both by that.

Question 6

Practice Molecular Question:
An organic substance containing 0.12g of Carbon, 0.02g of Hydrogen. The Mr of the compound is 56. Find the molecular formula.

Answer 6

Mr of compound = 56
Empirical formula = CH2
Mr of empirical = 14
Since Molecular formula = Mr of compound/Mr of empirical formula
Molecular formula = 56/14 = 4
So, we take the empirical formula and multiply that by 4.
CH2 x 4 =
C4H8

Question 7

Describe the process on how to calculate the molecular formula

Answer 7

Mr of compound(given in question)/Mr of empirical formula

Question 8

How do we know a chemical reaction has taken place?

Answer 8

You should reheat and reweigh until constant mass. We can see that if the mass changes that means there were still reactions happening. This is how we quantitatively know if a chemical reaction has taken place.

Question 9

How do you find the limiting reagent?

Answer 9

1. Write the balanced equation for the reaction
2. Calculate the moles of each reactant
3. Compare the moles & decide the limiting reactant(the smaller one)

Question 10

How do you find theoretical yield?

Answer 10

1) Find limiting reagent
2) Multiply the limiting reagent(in moles) by the coefficient of the thing you’re trying to find over the coefeccient of the limiting reagent.
Refer to this video:
https://youtu.be/9kO_EuC1Ki0?si=Nh9jwpMSdo7DPcbi

Question 11

What is the formula for Percentage Yield?

Answer 11

(Actual yield/Theoretical yield) x100

Question 12

Question: 40g of hydrogen was completely combusted and 280g of water was the product. Calculate percentage yield.

Hint(write the balanced equation to start)

Answer 12

Write balanced equation:
2H₂ + O₂ -> 2H₂O
Find moles in H₂, which is mass/mr so 40/2 = 20 moles
Find moles of H₂O by using molar ratio, since 2:2 = 1:1 its 20 moles in H₂O
Find mass by multiplying moles by Mr
20x18(mr) = 360
actual/theoretical x100 = percentage yield
280/360 x 100 = 77.777..%

Question 13

Define concentration

Answer 13

The amount of solute(the thing that gets dissolving) dissolved in a volume of solvent(the thing that does the dissolving).

Question 14

What can concentration also be referred to as(another word for it)

Answer 14

Molarity

Question 15

What is the formula for concentration?

Answer 15

Concentration = Moles/Volume

Question 16

State how to convert from cm³ to dm³

Answer 16

Divide by 1000 to switch from cm³ to dm³

Question 17

State the units for concentration and volume

Answer 17

Concentration = moles/dm³ or mol dm-³
Volume = dm³

Question 18

Calculate the number of grams of NaOH needed to make 50cm³ of NaOH with 2 mol/dm³.

Answer 18

moles = concentration x volume
50cm³ = 0.05 dm³(this is volume)
0.05dm³ x 2 mol/dm³ = 0.1 moles
mass = moles x mr
mr of NaOH = 40
mass = 0.1x40
mass = 4
Therefore, 4 grams of NaOH is needed to make 50cm³ of NaOH with 2 mol/dm³

Question 19

What is the formula for moles in a gas

Answer 19

moles = volume of gas(dm^3) / 24