Quantative Chemistry Flashcards
What is the law of conservation of mass?
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
Write a balanced equation of magnesium reacting with hydrochloric acid.
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
Define relative atomic mass and relative formula mass.
RAM - average mass of atoms in an element taking into account masses
and abundance of its isotopes, relative to 12C.
RFM - sum of RAMs of all atoms in the formula.
What is the relative formula mass of:
A) CaF²
B) C⁶H¹²O⁶
CaF² - (Ar values: Ca = 40, F = 19)
40 + 19 + 19 =
78
C⁶H¹²O⁶
- (Ar values: C = 12, H = 1, O = 16)
(12 x 6) + (1 x 12) + (16 x 6) =
180
The following reaction occurs in a test tube under a Bunsen
Burner:
4MgO(s) + CH4(g) → 4Mg(s) + 2H2O(g) + CO2(g)
The carbon dioxide and water escape from the test tube.
Use the equation to explain why
They are both gases
What is Avogadro’s constant?
The number of atoms, molecules or ions in a mole of a given substance.
The value of the constant is 6.02 x 1023.
What is the formula that links mass, molecular mass and moles
together?
Mass = Mr x Moles
What is the mass of 20 moles of calcium carbonate, CaCO3?
Mass = Mr x Moles
Mr = 100
100 x 20 = 2000 g
Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide.
Relative atomic masses (Ar):
carbon= 12, oxygen = 16
Moles = Mass / Mr
0.32 / 44 = 0.007
State what we mean by a limiting reactant in a chemical reaction
In a chemical reaction involving two reactants, it is common to use an
excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant because it limits the amount of products.
How much oxygen will be given off from 40.8 g of hydrogen peroxide?
Step 1: Write the balanced equation
2 H²O²(l) → 2 H²O + O²(g)
Mr of H²O² = 34
Step 2: Number of moles in 40.8 g : 40.8/34 = 1.2 moles
Ratio in the balanced equation of H2O2 : O2 = 2:1
Step 3 :Therefore number of moles of O2 = 0.6 moles
Step 4: Mass of oxygen =
0.6 x 32 (Mr of O2) = 19.2
Write down the two formulae that link concentration, mole/mass
and volume together.
Concentration (g per dm3) =
Mass (g)/Volume (dm3)
Concentration (mol per dm3)=
nr of moles/volume (dm3)
What is the molar volume of a gas at room temperature and pressure?
1 mole of a gas at room temperature and pressure occupies 24 dm3
What is titration?
A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration.
Why is it not always possible to obtain the theoretical amount of product in a chemical reaction? (3)
● The reaction may not go to completion because it is reversible.
● Some of the product may be lost when it is separated from the
reaction mixture.
● Some of the reactants may react in ways different to the expected
reaction (side reactions may occur).
How is the percentage yield of a product in a chemical reaction?
% Yield =
Actual mass of a product x 100%
—————————————————
Maximum theoretical mass of product
What is atom economy?
A measure of the amount of starting materials that end up as useful products.
It is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants.
What is the symbol for Relative formula mass?
Mr
How do you work out the Relative formula mass?
Adding the atomic masses of all the elements in the reaction
What are moles?
The measurements of chemical amounts
What is the relationship between the mole and relative atomic mass of any element
One mole is equal to the relative atomic mass of the element
How many molecules are in one mole of vinegar?
Avogadro’s constant (6.02 × 10²³)
Iron (Fe) has a relative atomic mass of 56. How much does one mole of iron weigh?
56 grams
How do you work out the number of moles from the mass and Mr
Moles = mass / relative formula mass
How do you calculate the RFM / Mr of an equation (3)
- Balance the equation
- Multiply the reactants and products by the amount of moles
- Add them up for an answer
What is the formula for working out the mass of a substance made when reacting with a given mass of another substance
Known mass / Mr (of known mass) × Mr (of unknown mass)
Explain how to use moles to balance equations
Step 1: write out the unbalanced symbol equation
Step 2: Calculate number of moles of each substance (mass / Mr)
Step 3: ratio the answer
Step 4: Balancing equation
