C3 Quantitative Chemistry Flashcards
how do you work out the relative formula mas of MgCl2
24 + (35.5 x 2) = 95
how do you work out the percentage mass of an element in a compound?
percentage mass of an element in a compound =
Ar x number of atoms in that element/ Mr of the compound x100
what is the Avogadro constant?
6.02 x 10^23
how do you work out the number of moles?
moles= mass/ Mr
how do you show that mass is conserved?
the total Mr on the left hand side of the equation is equal to the total right hand side of the equation so the mass is conserved.
what are the two explanations for a change of mass of an unsealed reaction vessel?
1) if the mass increases its because one of the reactants is a gas that found in air and all the products are solids, liquids or aqueous. before the reaction, the gas is floating around in the air. its there but contained din the vessel, so you cant account for its mass.when the gas reacts to form part of the product it becomes contained inside the reaction vessel so the total mass of the stuff inside the reaction vessel increases.
2) if the mass decreases , its because one or more of the products are gasses and all the reactants are solids, liquids are aqueous. before the reaction all the reactants were contained in the vessel. if the vessel inst enclosed, then any gasses can escape from the resction vesselas theyare formed. they are no longer contained in the reaction vessel, so you cant account for thier mass, the tottal mass of the stuff inside the reactin vessel decreases
how do you balance equations using reacting masses?
!) work out the Mr for each of the substances in the reaction
2) divide the mass of each substance by its Mr to calculate how many moles of each substance reacted or were produced.
3) divide each answer from step 2 by the smallest number of moles from step 2
4) if the numbers are nt whole number then multiply all the numbers by the same amount so they all become whole numbers.
5) write the equation out
the more solute there is in a given volume………..
the more concentrated the solution is
how do you calculate the concentration of a solution?
concentration = mass of solute/volume of solvent
what happens when one reactant is used up?
the reaction stops
what is the reactant called that get used up?
limiting reactant
the amount of product formed is ……………… to the amount of limiting reactant
directly proportional
how do you calculate the mass of a product formed in a reaction
1) write out the balanced equation
2) work out relative formula masses
3) find out how many moles there ll be of the other substance
4) use the number of moles to calculate the mass
