C4 Flashcards
What is relative formula mass symbol
Mr (r below m)
How do you find the relative formula mass of a compound
Add together relative atomic mass (number above symbol) of each element
Find the relative atomic mass of MgCl2
Mg + 2xCl = 24 + 2x35.5 = 95
So Mr of MgCl2 = 95
Why is mass always conserved in chemical reactions
Because no atoms are destroyed or created meaning there are the same number and types of atom of each side of a reaction equations
Explation of why you might observe a change in mass of an unsealed reaction vessel
If the mass increases, its probaly because of one of the reactants being a gas thats found in air. Before the reaction, the gas is floating in the air but its not contained in the reaction vessel so you can’t account for it’s mass. When the gas reacts to form part of the product, it becomes contained inside the reaction vessel meaning the total mass of the stuff inside the reaction vessel increases.
What is a reaction vessel
Reaction vessels are any vessels in which chemical reactants can be contained for a reaction to take place and under the needed physical conditions.
Why would a mass decrease?
Because one of the reactants is a gass and the other reactants are solids liquids or aqueous. If the vessel isn’t closed when reacting then the gas can escape meaning the products will loose mass
How to find the mass conserved in a reaction
Amount of element x relative formula mass (top number) added with other elements.
What is concentration
The amount of a substance (mass or number of moles) in a certain volume of a solution
The more concentration means the
More solute there is in a given volume
What is concentration measured in
G/dm cubed
Equation for concentration
Mass (g) / volume of solvent (dm cubed)
What is the concentration in g/dm cubed of a solution of sodium chloride where 30g of sodium chloride is disolved in 0.2 dm cubed of water
Concentration = 30/0.2 = 150 g/dm cubed
Steps on how to do the reacting mass calculations
- tick the balanced equation
- ? The unbalaced equation
- find moles of ticked = mass /mr
- find moles of ?
- find mass of ? = moles x mr
Mole:
A mole of any substance always contains the same number of atoms, molecules and ions
Moles from masses equation:
Number of moles = mass/Ar or mass/Mr
Masses from moles equation:
Mass (g) = number of moles x Ar
How are masses of atoms compared?
They are compared by measuring them relative to atoms of carbon 12
Why are chemical equations useful?
-for finding out how much of each substance is involved in a chemical reaction (use balanced symbol equation)
Limiting reactants
The reactant that gets used up first in a reaction
(NOT THE ONE IN EXCESS)
Example of limiting reactants:
-add dilute hydroclhloric acid to magnesium ribbon,
-hydrogen gas is given off and you see bubbles rise from magnesium until in excess
-the reason the reaction stops is that all the acid has been used up so the hydrocloric acid is the limiting reactant
How can you work out the limiting reactant?
From a balanced equation if you know the number of moles of reactants before you start
