quantitative chemistry

• what is the relative formula mass?

  the relative formula mass of a compound is the sum of the relative atomic masses of the atoms.
• how do you calculate moles?

  moles=mass/relative formula mass
• what is a limiting reactant?

  in a chemical reaction with 2 reactants you will ​often​ ​use​ ​one​ ​in​ ​excess​ ​to ensure​ ​that​ ​all​ ​of​ ​the​ ​other​ ​reactant​ ​is​ ​used. a limiting​ ​reactant:​ ​the​ ​reactant​ ​that​ ​is​ ​used​ ​up​ ​/​ ​not​ ​in​ ​excess​ ​(since​ ​it​ ​limits​ ​the amount​ ​of​ ​products
• what happens if a limiting reactant is being used?

  if a limiting reactant is being used, the amount of product being produced is restricted to the amount of excess reactant that reacts with the limiting one.
• ionic compounds are formed when metals and non metals react together
• metals form positive ions and most non metals form negative ions
• the charge of the ion depends on whether the element it metal or non metal and if its metal it has a positive charge, if its non metal it has a negative charge. the amount of ions depends on the elements group number. for e.g magnesium is a metal in group 2 and so it has a 2+ charge because its a metal and is in group two
• transition metals can form several ions such as iron: fe2+(iron ll) and fe3+ (iron lll)
• in an ionic compound the charges on the ions have to cancel out to leave an overall charge of zero
• you are never allowed to change the small numbers in a chemical formula as this produces a different molecule
• what is the relative atomic mass?

  the relative atomic mass of a element is the average mass of the isotopes of that element weighted to take into account the abundance of each isotope
• relative formula mass has no units
• relative formula mass never involves big numbers
• how do you calculate the relative formula mass of CaSO4? ar Ca=40 ar S=32 ar O=16

  ans- mr=136

  
• how do you calculate mass of a number of moles?

  mass=number of moles x relative formula mass (Mr)
• calculate the number of atoms in 56g of calcium oxide? Ar Ca=40 Ar O=16


  you do mass/ the Ar and then multiply by avagadros constant which is 6.02 x 10^23 = 6.02 x 10^23