4.3- Quantitative Chemistry Flashcards
(28 cards)
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.
What is the relative formula mass?
Relative molecular (formula) mass is the sum of all the relative atomic masses added up in the ratios shown in the chemical formula of a molecule. (Mr)
What is the relative atomic mass?
Relative atomic mass is the average mass of an element compared to one-twelfth the mass of a single carbon-12 atom. (Ar)
How do you calculate relative formula mass?
Relative molecular (formula) mass is the sum of all the relative atomic masses added up in the ratios shown in the chemical formula of a molecule. (Mr)
What would be the relative formula mass of MgCl2
Magnesium atomic mass = 24
Chlorine atomic mass = (35.5)x 2 =71
Mr= 71 + 24
The relative formula mass of magnesium chloride is 95
How do you calculate the percentage mass of a certain compound?
% mass = (atomic mass of element / formula mass of the compound) x 100
What is the percentage mass of Ca in CaCO3?
(40/100) x 100 = 40%
Why might some reactions appear to have a change in mass?
Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account.
For example: when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal or in thermal decompositions of metal carbonates carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product.
How do you calculate uncertainity?
Uncertainty = range /2
What is a mole?
One mole of any substance is simply the relative molecular or atomic mass of a substance expressed in grams.
What is Avogadro’s constant?
1 mole = 6.02 x 10 to the power of 23 (Avogadro’s constant)
1 mole of a substance has a mass which is equivalent to its relative molecular mass in grams.
Equation for moles?
Number of moles = mass(g) / relative molecular mass
What do balanced equations tell us in terms of moles?
Balanced equations tell us the molar ratio. A molar ratio is the proportion of moles of one substance to the moles of another substance in a chemical reaction.
For example:
N2 + 3H2 →2NH3
Here, 1 mole of nitrogen reacts with 3 moles of hydrogen to make 2 moles of ammonia. They have a 1:3:2 ratio.
How do you use the molar ratio to calculate masses of reactants or products?
- Calculate the number of moles of the given substance
- Use ratio to calculate the number of moles of the substance they are asking us about
- Answer the question
What is the limiting reagent?
More often than not, when carrying out a chemical reaction, you do not use the exact amount of products as predicted in the balanced chemical equation. One of the reactants will be in excess.
The reactant that gets used up first in the reaction is called the limiting reactant or reagent.
What does concentrated mean?
A more concentrated solution has a higher amount of solute in the same volume solution than a less concentrated solution.
Equation for concentration?
Concentration (g/dm3)= mass/amount of solute (g)/ volume of solution (dm3).
(1dm3 = 1000 cm3).
What is the percentage yield?
The mass of product that a chemical reaction produces is called the yield. However, it is more helpful to think about this as the percentage yield, which compares the expected yield with the actual yield.
How do you calculate percentage yield?
% yield = (actual mass of product formed / maximum theoretical mass of product possible) x 100%
Example: A factory processes 200 tonnes of limestone a day. It collects 98.0 tonnes of calcium oxide, the useful product. What is the percentage yield of the reaction in the kiln, assuming limestone contains only calcium carbonate.
CaCO3 → CaO + CO2
Mr of CaCO3 = 40 + 12 + (16 x 3) = 100
Mr of CaO = 40 + 16 = 56
The balanced symbol equation tells us that 100 tonnes of calcium carbonate makes 56 tonnes of calcium oxide. Therefore, 200 tonnes of calcium carbonate would make 56 x 2 = 112 tonnes of calcium oxide (theoretically).
% yield = (98.0 / 112) x 100 = 87.5%
What are some factors affecting percentage yield?
- The reaction may be reversible- as products form, they react to reform the reactants again.
- Some reactants may react to give unwanted or unexpected products in alternative reactions.
- Some of the product may be lost in handling, or be left in the apparatus.
- The reactants may not be pure.
- Some of the desired product may be lost when being separated from the reaction mixture.
What is atom economy?
The atom economy of a reaction is a measure of the extent to which the atoms in the starting materials (reactants) end up in the desired product.
Why is atom economy important?
Atom economy is important in industrial processes because we need to conserve the Earth’s resources and minimise pollution.
How do you calculate atom economy?
% atom economy = (relative formula mass of the desired product / sum of relative formula masses of reactants from equation) x 100
