Quantitative Chemistry Flashcards
How do you calculate relative formula mass ( Mr )
work out how many atoms of each element are there in the chemical formula then add the relative atomic masses ( Ar )
What is the equation that links no. of moles, mass and rfm (Mr)
No. of moles = Mass (g) / Relative formula mass (Mr)
Use this for compounds
What is the equation that links no. of moles, mass and relative atomic mass (Ar)
No. of moles = Mass (g) / Relative Atomic Mass (Ar)
Use this for elements
Here is an example of reacting mass question. Understand the ratio method!
Example
12 g of magnesium reacts completely with excess hydrochloric acid to form magnesium chloride and hydrogen:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Calculate the maximum mass of hydrogen that can be produced. (Ar of Mg = 24, Mr of H2 = 2)
Amount of magnesium = Mass/(Relative Atomic Mass)
Amount of magnesium = 12/24
= 0.5 mol
Looking at the equation, 1 mol of Mg forms 1 mol of H2, so 0.5 mol of Mg forms 0.5 mol of H2
Mass of H2 = Mr × amount
= 2 × 0.5
= 1 g
How do you work out percentage mass
Total Ar of the element
Percentage Mass= ——————————— * 100
Mr of the Compound
What is the theoretical yield of a reaction
result of a rection that is worked out with mole calculations
Why in reality is the theoretical yield never produced
- The reaction may not go to completion
- Reactants may be impure
- If it is a gas some could escape
- Some reactions may be reversible
- A side reaction may occur
What is the equation for % yield
original mass obtained
% yield = ———————————— * 100
theoretical mass
What is percentage atom economy
efficiency of a chemical process in terms of all atoms involved and the desired products produced.
What is the equation for percentage atom economy
RFM of Wanted Product
% atom economy = ————————————– * 100
Total RFM of All Products
How do you calculate moles in solution using g/dm^3
Mass (g)
Conc. of solutions (g/dm^3) = ————————-
Volume (dm^3)
How do you calculate moles in solution using moles/dm^3
Moles
Conc. of solutions (mol/dm^3) = ————————-
Volume (dm^3)
What is the conversion between cm^3 , dm^3 and m^3
Divide 1000 divide 1000
Cm3 —————–> dm3—————–>m3
*1000 *1000
What is the process of titration
Finding out the concentration of a solution by finding out at what volume it will neutralise after adding a standard (known) solution
What is the Law of Conservation of Mass
Key Concept: No atoms are lost or made during a chemical reaction so the:
’ mass of products = mass of the reactants’
What are ions
atoms with a charge
How do ionic compounds form
when metals react with non metals
What do metals and most non metals form
metals form positive ions, non metals form negative ions
Except H+ and NH4+
What is charge dependent on
Group number; except transition metals
What is Avogadro’s Constant
1 Mole is 6.02x10^23
How can Avogadro’s Constant be used
work out the number of atoms in a specific amount of a compound
e.g. Calculate the no. of atoms in 48g of magnesium. Ar of Magnesium= 24
No. of moles = mass/Mr
Moles = 48/24=2 moles
2 moles = 2 * 6.02x10^23 no. of atoms = 1.204x10^24
Define a limiting reactant
the reactant that is all used up is called the limiting reactant - it sets a limit on how much the excess product can form.
How do you use limiting reactants in calculation
use the limiting reactant to calculate the mass of the product
How much volume does 1 mole of any gas occupy
a volume of 24dm3 ( at room temp. and pressure)
room temp. = 20 C Room pressure = 1 atmosphere
What is the equation for volume of a gas
Volume (dm^3) = No. of moles * 24
