topic 3 - quantitative chemistry Flashcards
what does the law of conservation of mass state?
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 are ions?
atoms with a charge
how are positive and negative ions formed?
- metals form positive ions and non-metals forms negative ions
what are positive non-metal ions?
- hydrogen ion
- ammonium ion
what is the rule for ionic compounds?
- in an ionic compound, the charges on the ions have to cancel out to leave an overall charge of zero
how do you calculate the %mass of an element in a compound?
percentage mass of an element in a compound = Ar x number of atoms of that element / Mr of the compound
x 100
how do you calculate the number of moles?
mass (g) / Mr
what is Avogadro’s constant?
- the number of atoms, molecules or ions in a mole of a given substance.
- the value of the constant is 6.02 x 10^23
Calculate the number of atoms in 28g of lithium. Ar Li = 7
number of moles = 4 moles
number of atoms = 4 x 6.02 x 10^23
number of atoms = 2.408 x 10^24
what does a limiting reactant in a chemical reaction mean?
- In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used
- the reactant that is completely used up is called the limiting reactant because it limits the amount of products
what is meant by concentration?
- concentration tells us the mass of a solute in a given volume of solution
what is a solute?
a solute is a chemical that is dissolved in a solvent
what is often used as a solvent in chemistry?
water
what is the unit for concentration?
one unit of concentration is g/dm^3
what is the equation for concentration?
concentration (g/dm^3) = mass (g) / volume (dm^3)
what happens if we increase the mass of a solute?
- if we increase the mass of a solute and keep the volume the same, then we increase the concentration
what happens if we increase the volume of the solution?
- if we increase the volume of solution and keep the mass of the solute the same, then we decrease the concentration
why is it not always possible to achieve 100% yield in a chemical reaction?
- some of the product may be lost when it is separated from the reaction mixture
- some of the reactants may react in different ways to the expected reaction so we do not get the product we expect
- reversible reactions may not go to completion
what is the equation for percentage yield?
percentage yield = actual yield / theoretical yield x 100
why do industries aim for a high percentage yield?
- lower costs and less waste
what is atom economy?
atom economy is a measure of the amount of starting materials that end up as useful products
what are two reasons why it is important to have a high atom economy?
- less waste is produced
- more sustainable as less raw materials are used up
what is the equation to calculate atom economy?
atom economy = Mr of desired product / total Mr of reactants x100
what is meant by concentration? (T.S.)
- concentration tells us the number of moles of a solute in a given volume of solution
what is the unit of concentration? (T.S.)
- one unit of concentration is mol/dm^3
what is the equation linking concentration, moles and volume?
concentration = number of moles / volume
what is the volume of one mole of any gas at room temperature and pressure?
24dm^3
what is room temperature?
20°C
what is room pressure?
1 atmosphere
what is the equation to calculate the volume of a gas?
volume (dm^3) = number of moles x 24
