Topic 4 - Chemical Calculations Flashcards
What do we use to compare the masses of elements?
Relative atomic masses (Ar)
If the relative formula mass of Hydrogen = 1, Sulfur = 32 and Oxygen = 16, what is the Relative Formula Mass of H2SO4?
H2 = (1x2) + S (32x1) + O4 (16x4) = 98.
What is a mole?
Relative Formula Mass in Grams.
What is the avogadro constant?
The number of atoms in a mole which is constant at 6.02 x 10 to the power of 23.
What is the equation for Moles?
Moles = Mass / Mr.
What is the equation “triangle” for mass, moles and Mr?
Mass
Moles x Mr
Balance the equation - H2 + Cl2 -> HCl
H2 + Cl2 —> 2HCl
How many moles of NaOH do I have if I have 100g of it and it’s relative formula mass is 40?
100/40 = 2.5
What is the equation for percentage yield?
Percentage Yield = (Actual Mass of Product / maximum theoretical product possible) x 100
What is the equation for atom economy?
Percentage atom economy = (relative formula mass of desired product in the equation / sum of the relative formula mass of the reactants from the equation) x 100
What is the equation for concentration (g/dm3)?
Mass of solute (g) / volume of solution (dm3)
What is a titration?
Where you measure the exact volumes of a strong acid and strong acid to make the solution neutral. The point at which they have reacted is called the end point of the reaction. You can judge this using universal indicator.
What is the method for carrying out a titration?
1) Measure a known volume of alkali into a conical flask using a volumetric pipette. Before doing this you should wash your pipettes with distilled water and then some of the alkali.
2) Add a few drops of universal indicator and mix it with the alkali.
3) Rinse a burette with distilled water and then some of the acid, allowing some acid to pass through the tap. Then pour the acid into the burette. The burette has markings every 0.1cm3 to allow this.
4) Record the reading on the burette then open the tap to release a small amount of acid into the flask. Swirl the flask to make sure the two solutions are mixed.
5) Repeat step 4 until the indicator turns green to indicate neutrality. Record the reading on the burette and work out the volume of the acid that has run into the flask. This volume is known as a titre.
6) Repeat the process at least three times, Discard any anomalous result. Alternatively, keep repeating until the titration gets you two results within 0.1cm3 of each other. These precise results are called concordant. Then calculate a mean value to improve accuracy.
7) Using your results you can calculate the concentration of alkali in mol/dm3.
What is the equation for number of moles of gas?
Moles of gas = volume of gas (dm3) / 24dm3 or volume of gas (cm3) / 24000cm3.
In a titration, what is used to measure a volume of acid or alkalo?
Pipette.
