SC9 - Calculations Involving Masses ✓ Flashcards
SC9a - What is the difference between emirical and molecular forumla?
- Molecular formula is the actual amount of atoms of each element in a compound
- Empirical formula is the simplest whole number ratio of atoms of each element in a compound
SC9a - How do you work out the empirical formul of a compound from the masses of each element?
- Find the moles by doing mass/Mr
- Divide both of the molar values by the smalles of the two
- e.g: 10g of Ca and 17.8g of Cl 10/40 = 0.25 17.8/35.5 = 0.5 0.25/0.25 = 1 0.5/0.25 = 2 1:2 ratio
SC9a - Describe how you can find the empirical formula of magnesium oxide by heating magnesium ribbon.
- Sand and Weigh the Magnesium: Clean the magnesium ribbon with sandpaper and then weigh it to find its mass.
- Weigh the Crucible and Lid: Get the mass of the empty crucible with its lid on.
- Weigh Magnesium in the Crucible: Put the magnesium in the crucible, weigh it, and subtract the weight of the empty crucible to find the mass of just the magnesium.
- Heat the Magnesium: Heat the magnesium in the crucible with a Bunsen burner, occasionally lifting the lid to allow oxygen in for the magnesium to burn and form magnesium oxide.
- Weigh the Magnesium Oxide: After the reaction, weigh the crucible with the magnesium oxide, subtract the weight of the empty crucible, and you have the mass of the magnesium oxide.
- Find the Mass of Oxygen: Subtract the starting mass of magnesium from the mass of magnesium oxide. This difference is the mass of oxygen that reacted with the magnesium.
- Calculate the Empirical Formula: Use the masses of magnesium and oxygen to find the ratio of moles of magnesium to moles of oxygen. This mole ratio, simplified to the smallest whole numbers, gives you the empirical formula of MgO.
SC9c - What is the molar mass formula?
mol = mass/Mr or Ar (relative atomic or formula mass)
SC9b - how do you work out the concentration of a solution?
Concentration (g dm-³) = mass of solute (g) ÷ volume of solution (dm³)
SC9b - What is a dm³ equal to?
1L = 1000 cm³
SC9b - What is the conservation of mass and what does it depend on?
- The idea that the mass of the reactants = the mass of the products in a closed system.
- In an open system, gases and liquids can sometimes escape altering this
SC9b - How do you calculate the mass of a reactant needed to make a product (or vice versa)
- Work out the mols in the one you’ve been given the mass for and divide this by the big number next to it
- Multiply this by the big number next to the one you are trying to work out to give you the mols in this
- Multiply this by its Mr to get the mass
- Example: 2Al + 3Cl₂ → 2AlCl3
- How many grams of Cl₂ to form 53.4g of AlCl3
- AlCl3 has an Mr of 133.5 so do 53.4 ÷ 133.5 = 0.4
- As the ratio of big number is 3:2 do 0.4 ÷ 2 and then x 3 to get 0.6 Do 0.6 x Cl₂s Ar whcih is 71 to get 42.6g
SC9c - What is Avogadro’s constant?
The amount of particles in a mole 6.02x1023
SC9c - What is a limiting reactant?
In a chemical reaction, the limiting reactant is the substance that gets used up first, limiting the amount of product that can be made because the reaction can’t happen without it.
SC9c - How do you work out the limiting reactant?
- Firstly work out the balanced equation of the reaction (This may already be given to you)
- The big numbers show the ratio of mols needed
- Using the mass and Mr, work out the mols you have of one of them
- From here, work out the mols you need for the other one for the second one to not be the limiting reactant
- Then work out how many mols you have of the second one.
- If it is more than you would’ve needed then it is in excess if not then it is the limitjng reactant
- Use the values for the actual amount of mols used up to figure out the masses that will actually be used up
SC9c - What is stochiometry?
The ratio of moles is a reaction
