5 - Formulae, Equations & Amounts Of Substances Flashcards
(27 cards)
What are the two types of errors and how can they be avoided?
- Systematic - often caused by equipment or procedural errors, continued across all results. To avoid, use better equipment
- Random - often caused by human error (e.g. Reading burette incorrectly), varying between results. Overcome by repeating experiments (improving accuracy and removing outliers).
In titrations, what are 3 possible errors as a result of poor technique?
- Not thoroughly shaking volumetric flask after topping up to 250cm^3
- Getting air below the tap in the burette
- Getting air in the stem of pipettes
- Dripping solution from burette too quickly, overshooting the end point
- Not swirling the flask after each drop of solution from the burette is added
What is the formula for percentage uncertainty?
(Uncertainty/Difference in readings) x 100
How do you work out the uncertainty for a certain piece of equipment?
Half the smallest increment the equipment is measurable to - e.g. A burette with increments of 0.1cm^3 has an uncertainty of +/- 0.05cm^3
What are two ways in which % uncertainty can be minimised?
- Use more precise equipment with lower uncertainties
* Use larger volumes of solution
How do you find the actual total uncertainty in a titration reaction?
- ) Find individual % uncertainties for each bit of equipment
- ) Add the percentages together
- ) Find that percentage of the end result of the titration
What is the formula for percentage yield?
% yield = (actual yield/theoretical yield) x 100
What is atom economy?
The proportion of reactants that become useful products
What is the formula for atom economy?
Atom Economy = (Mass of desired product/Sum of the masses of all products) x 100
What is the atom economy in addition reactions, where only one product is formed? And why?
The atom economy is 100%, because there are no by-products
What is a mole?
The amount of a substance containing the same amount of particles as in 12g of carbon 12
How do you work out the number of particles?
Moles x Avogadro constant
How do you work out mass?
Mass = moles x RMM
How do you work out concentration?
Conc. = moles/volume (dm^3)*
- if in cm^3, Divide by 1000
How do you work out the empirical formula from percentage composition?
- ) calculate miles of elements: percentage/mass
- ) Divide all by smallest number to get ratio
- ) use ratio to find how many atoms of each element there are, giving empirical formula
How do you work out the empirical formula from combustion masses?
- ) Find moles using masses
- ) multiply moles by amount of element in compound
- ) Divide moles by smallest to get ratio
How can you use the empirical mass to work out the molecular?
Molar mass/empirical mass = n
Multiply compound by n to get Molecular mass
What are displacement reactions?
A more reactive element replacing less reactive element in a compound
What does an acid/base reaction form?
Acid + base -> salt + water
What is a precipitation reaction?
2 aqueous compounds reacting to form solid precipitate
What is the gas triangle?
Volume = moles x 24,000cm^3
What experimental setups can you use to collect gas?
- gas syringe and conical flask
* conical flask and inverted measuring cylinder in water
What is the ideal gas equation?
pV = nRT
pressure x volume = moles x 8.31 x temperature
What is the method for an acid-base titration?
- ) rinse burette and pipette and conical flask with distilled water
- ) fill burette with standard solution
- ) open tap to remove air bubbles. Read to 0.05cm3
- ) Add indicator to beaker with base or acid in
- ) do rough titration where burette solution dropped quickly into beaker
- ) repeat at least 2 more times with dripping burette to get concordance titres. Average these to get mean titre
