Quantitative Chemistry Flashcards
Conservation of mass and balanced chemical equations
What is the law of conservation of mass?
No atoms are lost or made in a chemical reaction so the mass of the products equals the mass of the reactants
Conservation of mass and balanced chemical equations
Describe what will happen to the mass in a closed system reaction
Since all the reactants and products stay in the beaker, the total mass of the beaker and the substances in it stay the same during the reaction
Conservation of mass and balanced chemical equations
Describe what will happen to the mass in an open system reaction
If gas escapes, the total mass will look as if it has decreased. If gas is gained, the total mass will look as if it has increased
Conservation of mass and balanced chemical equations
How do you balance an equation?
- Count how many atoms of each type are on each side of the equation
- If the numbers are the same then the equation is balanced
- If the numbers are not the same, then numbers are put in front of the formulas (this adds more than that substance). You cannot change the formulas (this would make a different substance)
- Keep doing this until the equation is balanced
Relative formula mass
What is the relative atomic mass?
The Ar is the relative mass of its atoms compared to the mass of a carbon-12 atom. Since atoms have very little mass, their relative atomic masses are used. (mass number)
Relative formula mass
What is the relative formula mass?
The Mr is the sum of the Ar’s of the atoms in the numbers shown in the formula
Relative formula mass
Calculate the Mr of calcium hydroxide (Ca(OH)2)
Ar of Ca= 40, Ar of O= 16, Ar of H= 1
Mr= Ca + O + O + H + H = 40 + 16 + 16 + 1 + 1 = 74
Relative formula mass
What is the equation for percentage by mass?
% mass = (total Ar of the element / Mr of the compound) x 100
Mass changes when a reactant or product is a gas
The following 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 escapes from the test tube
Use the equation to explain why
They are both gases
Chemical measurements
What is uncertainty and why does it occur?
Whenever a measurement is made in science, there is always some uncertainty in the result obtained.
Causes include:
- its difficult to judge whether (for example) a thermometer is showing 18, 18.5 or 19
- its difficult to judge exactly when a chemical reaction has finished
Chemical measurements
How can we estimate the uncertainty?
- +/- half the resolution
- +/- half the range
Moles
What is Avogadro’s constant?
The number of atoms, molecules or ions in a mole of a substance.
602,000,000,000,000,000,000,000 / 6.01 x 10^23
1 mole = avogadro’s number
Moles
How do you calculate the number of particles in something?
Number of particles = avogadro’s constant x the amount of substance in mol
Moles
How do you calculate the number of moles?
number of moles = mass / relative formula mass
Amounts of substances in equations
How do you calculate the mass of a substance in a balanced symbol equation?
- Find the moles of the substance you know the mass of with n=m/Mr
- Ratio of moles - what we know : what we want to find out
- Find the mass os the substance you want to find out with n=m/Mr
Limiting reactants
What is the limiting reactant?
The reactant that is completely used up is called the limiting reactant because once all of that reactant is gone, the reaction stops
Limiting reactants
How do you find out if a reactant is the limiting reactant?
- Find the moles of the reactant we want to prove as the limiting reactant
- Find the moles of another substance in the equation
- Do the ratio of them
- If the moles calculated is less than the moles of the ratio, it is the limitng reactant
Concentration of solutions
How do you convert cm3 to dm3?
/ by 1000
Concentration of solutions
How does a solution form?
A solution forms when a solute (solid) dissolves in a solvent (liquid)
Concentration of solutions
How do you calculate the concentration of a solution?
concentration = mass of solute / volume
Percentage yield
What is % yield and how do you calculate it?
The amound of product made is the yield
% yield = (mass of product actually made / maximum theoretical mass of product) x 100
Atom economy
What is atom economy and how do you calculate it?
The atom economy is a measure of the amount of starting materials that end up as useful products
atom economy = (relative formula mass of desired product / sum of relative formula masses of all reactants (or all products)) x 100 if wanted as a %
Using concentrations of solutions in mol/dm3
What is the equation to calculate the number of moles?
number of moles = concentration x volume
