Chemistry OCR A Level Chapter 3 - Amount of Substance

How do you convert degrees Celsius to Kelvin?

+273

What is 1ml equal to?

- 1 cm^3

What is 1l equal to?

- 1 dm^3

What assumptions are made when calculating an experimental formula for a hydrated salt?

-1. that all the water has been lost. A good solution is so heat to constant mass - the crystals are reheated repeatedly until the mass of the residue no longer changes, suggesting that all the water has been removed. -2. that there is no further decomposition. Many salts decompose further when heated; for example, if heated very strongly, copper(II) sulfate decomposes to form black copper(II) oxide. This can be very difficult to judge if there is no colour change.

What is a standard solution?

-A standard solution is a solution of known concentration. -Standard solutions are prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume.

What are Hydrated salts.

-Many coloured crystals are hydrated - water molecules are part of their crystalline structure. -This water is known as water of crystallisation. -The water of crystallisation is shown by a large dot between the compound formula and the units of water.

What are the values of RTP?

-RTP is about 20 degrees celcius and 101kPa (1 atm) pressure. -At RTP, 1 mole of gas molecules has a volume of approximately 24.0dm^3. -Therefore, at RTP, the molar gas volume = 24.0dm^3mol^-1.

What is the relative formula mass?

-Relative formula mass compares the mass of a formula with the mass of an atom of carbon-12. -It is calculated by adding together the relative atomic masses of the elements in the empirical formula.

What is the relative molecular mass?

-Relative molecular mass, Mr, compares the mass of a molecule with the mass of an atom of carbon-12. -You can easily calculate a relative molecular mass by adding together the relative atomic masses of the elements making up a molecule.

What is the atom economy of a chemical reaction?

-The atom economy of a chemical reaction is a measure of how well atoms have been utilised. -Reactions with high atom economies: -produce a large proportion of desired products and few unwanted waste products. -are important for sustainability as they make the best use of natural resources. -Atom economy is based solely on the balanced chemical equation of a reaction and assumes a 100% yield. -Atom economy = (sum of molar masses of desired products / sum of molar masses of all products) x 100%

What is the limiting reagent?

-The limiting reagent is the reactant not in excess and it will be completely used up first and will stop the reaction.

What is a non-hydrated salt called?

-anhydrous.

What is the amount of substance measured in?

-moles. -one mole is the amount of substance that contains 6.02x10^23 particles. -The Avogadro constant, N(subscript A) is 6.02x10^23mol-1, the number of particles in each mole of carbon-12. -12g of carbon-12 contains 6.02x10^23 atoms. -The mass of one mole of atoms of any element is the relative atomic mass in grams,

What is the ideal gas equation?

-pV = nRT -The ideal gas constant R is a pV=nRT constant and always has the value of 8.314Jmol^-1K^-1 -Temperature is in units of K (Kelvin), which starts at absolute zero (-273 degrees Celsius). Each 1K rise in temperature is the same as a 1 degree rise in temperature. -Volume is measured in m^3. (divide dm^3 by 1000) -Pressure is in Pascals (Pa)

Why is the empirical formula needed?

-the empirical formula is important for substances that do not exist as molecules. This includes metals, some no-metals (e.g. C, Si), and ionic compounds (e.g. NaCl) -these substances form giant crystalline structures of atoms or ions. It would be impossible to base a formula on the actual number of atoms or ions - the numbers would go into billions of billions and would vary depending on the size of the crystals. -the empirical formula is the ratio of atoms or ions in the structure and will always be the same.

Why is the theoretical yield difficult to achieve?

-the reaction may not have gone to completion. -other reactions (side reactions) may have taken place alongside the main reaction. -purification of the product may result in the loss of some of the product.

How do you calculate empirical formula from mass?

Step one - Convert mass into moles. Step two - To find the smallest whole-number ratio, divide by the smallest whole number. Step 3 - Write the empirical formula.

How do you convert between amount in moles and gas volumes?

amount n (mol) = volume V / molar gas volume

What equation links amount of substance, n , mass, m, and molar mass, M?

amount n = mass m / molar mass M

What equation connects amount of substance, n, volume, V, and concentration, c?

n = c x V

The conversion of starting materials into a desired product is expressed by the percentage yield. How do you work out percentage yield ?

percentage yield = (actual yield / theoretical yield) x 100%