P1- Topic 3-Quantitative chemistry 2022 Flashcards
What is the law of conservation of mass? 2
no atoms are lost or made during a chemical reaction
the mass of the products
equals the mass of the reactants
Define relative atomic mass and relative formula mass. 2
RAM - average mass of atoms in an element taking into account masses
and abundance of its isotopes, relative to ¹²C
RFM - sum of RAM’s of all atoms in the formula.
What is the relative formula mass of CaF₂
Ar values: Ca = 40, F = 19
40 + 19 + 19 = 78
Do you use big numbers when calculating relative formula mass
no
Reaction occurs in a test tube under a Bunsen
Burner:
4 MgO(s) + CH4(g) → 4 Mg(s) + 2 H2O(g) + CO2(g)
The carbon dioxide and water escape from the test tube.
Use the equation to explain why.
They are both gases
how you could increase the precision of the results
Measure to more decimal places (higher resoloution)
use a more sensitive balance /apparatus
what is the equation for the percentage by mass of an element ?
total RAM of atoms of that element
——————————— *100
RFM of the compound
What is Avogadro’s constant? 2
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
moles = mass(g)/ RAM or RFM
calculate the number of atoms in 1 mole of hydrogen chloride
1 atom of hydrogen and chlorine so two atoms in total
6.0210²³ molecules of hydrogen chloride
so 2 x 6.0210²³
State what we mean by a limiting reactant in a chemical reaction 4
In a chemical reaction involving two reactants an excess of one of the reactants is used to ensure that all of the other reactant is used.
completely used up= limiting reactant because it limits the amount of products.
the other is in excess
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 meant by concentration
the mass of a solute in a given volume of soloution
what gives a higher concentration
a smaller volume or larger mass of solute
what gives a lower concentration
a larger volume or smaller mass of solute
Why is it not always possible to obtain the theoretical amount of product in a chemical reaction? 3
-reaction not go to completion= 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
what is the equation for percentage yield
% Yield =
Actual mass of a product
——————————— x 100%
Maximum theoretical mass of product
What is the % yield of NH₃
if 40.5 g NH₃ is produced from 20.0 mol H₂ and excess N₂?
Step 1 - Write the balanced equation
N₂+ 3 H2 → 2 NH₃
Step 2 - Calculate the theoretical amount of NH₃
Moles NH₃ ratio of H₂ to NH₃ is 3:2); of 20/1.5 = 13.3 moles
13.3 X 17 (Mr of NH₃)=227
Step 3 - Calculate percentage yield of NH3
40.5/227 x 100 = 17.8%…
What is atom economy?
A measure of the amount of starting materials that end up as useful products
what does from the equation mean in atom economy?
the big numbers count so it must be balanced
what is the equation for atom economy?
RFM of desired product(from equation)
——————————–*100
Sum RFM masses of all reactants from equation
reasons why a particular reaction pathway is chosen/not chosen(atom economy)5
atom economy yield rate equilibrium position usefulness of by-products
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 the equation for volume of gases?
volume= numer of moles*24
dm³
