2.1.3 Amount of substance Flashcards
What is the formula for calculating the percentage of an element in a compound?
% of an element = (number of atoms X relative atomic mass ÷ Mr) x 100
What is the equation for working out the number of atoms/ions/molecules in a substance?
Number of atoms/ions/molecules = Moles x 6.02x10²³
What is the method for calculating the empirical formula?
- Find the mass of both substances (usually given in the question)
- Calculate the moles of both substances
- Divide the mole values by the smallest value
- Adjust the ratio. If the compound is out by 0.01, it can be rounded. If it isn’t, the compound must be scaled up.
- Form the formula from the ratio
Non-required practical
Describe the method for calcualting the amount of water in a hydrated salt.
Part 1 - practical
* Weigh the crucible and record it’s weight
* Add a known amount of the salt. Weigh the crucible and salt together.
* Heat the hydrated salt with a bunsen burner (on a blue flame) for a few minutes, it will turn white.
* Weigh and continue to heat the salt until the mass stays constant.
Part 2 - calculation
* (Anhydrous salt + crucible) - crucible
* (Hydrated salt + crucible) - (anhydrous salt + crucible)
* These 2 values are the masses which can be subsituted into the emprical formula method.
What is the equation for molar gas volume?
Moles (n) = volume (dm³) / 24
24,000 should be used if the volume is given in cm³
What is the ideal gas equation?
pV = nRT
- P = Pressure (Pa)
- V = volume (m³)
- n = moles of gas (mol)
- R = ideal gas constant (8.314Jmol⁻¹K⁻¹)
- T = temprature (K)
How is Kelvin (K) converted into ℃?
K - 273
How is KPa converted into Pa?
Take the KPa value and add x10³ onto it
What is the equation for concentration?
Concentration = (moles X 1000) ÷ volume
What is the equation for % yield?
(Actual moles ÷ theoretical moles) x100
What is the equation for atom economy?
(Mr of desired products ÷ Mr of all products) x100
