Moles Flashcards
(46 cards)
Moles equation
Moles = volume/ concentration Moles = mass/ mr
What is moles equation used for?
Solids
Gas equation
Pressure x volume = no. Moles x gas constant x temp
PV=nRT
Mass equation
Mass= Mr x moles
What unit is mass in?
Grams
What is Avogadro’s constant?
6.02 x 10 to the power of 23
How to find mass (steps)
1) find moles (mass/mr)
2) ratio
3) find mass (mr x moles)
What to do is there is 2 masses given?
1) Work out moles of each
2) work out ratio and which one is in excess
3) mr x moles to work out mass
What do you need to make a solution?
Solid - the solute
Liquid - the solvent
Known concentration practical the solid
What do you need to know ?
- mass
- use a balance
- Mr
- calculate the moles
Known concentration practical - how to weigh by difference?
- put balance to zero
- put weighing boat onto balance and record mass (m1)
- add rough amount of solid to boat and record mass (m2)
- add solid mass to beaker and record mass of boat again (m3)
- m2-m3=exact mass of solid added
Known concentration practical -dissolving the solid
- use small beaker
- add solid to the beaker
- add approx 50cm3 of distilled water
- stir until full dissolved
Why do we use distilled water when dissolving the solid?
So the water doesn’t have any other chemicals and results aren’t inaccurate
Known concentration practical
-what do we know about the liquid?
- the volume
- use a 250cm3 volumetric flask (always state size)
Know concentration - how to work out liquid?
- transfer the 50cm3 from beaker into flask using funnel
- rinse beaker with distilled water and add the washings
- slowly add water until near the meniscus
- use as dropper to add water until meniscus is level with the line (250cm3)
- place stopper on top and invert 10 times to mix the solution thoroughly
Things to include in known concentration results table
- mass of empty boat
- mass of boat + solid
- mass of boat after solid
What is volume measured in?
Dm3
Concentration calculation
Concentration=moles/volume
What is uncertainty measured in?
%
Uncertainty calculation
(Error value of equipment/value measured) x 100
Testing known concentration (acid to alkali method)
- use pipettes and pipette filler to add 25cm3 of sodium hydroxide solution to clean conical flask
- add drops of indicator and put the conical flask on a white tile
- fill the burette with hydrochloric acid and note starting volume
- add acid from burette to alkali in conical flask,swirl to mix
- stop adding acid when indicator first permanently changed colour and note final volume
- calculate volume of acid added to neutralise alkali (titre value)
- repeat steps 1-5 until concordant titres are obtained (more accurate results are obtained of acid is added drop by drop near end point)
How many times do you test the known concentration?
A rough time, then 1,2 and 3 as u need 2 results that are within 0.1 of eachother
Titration practical method
- weigh it accurately about 1.3 g of anhydrous sodium carbonate and make it up to 250cm3 of solution in volumetric flask. Record masses in table
- titration 25.0 cm3 portions of this dilution against hydro caloric acid until concordat result are achieved. Use methyl orange as indicator. Record results in table.
Uncertainty for balance
2 x 0.000.5g/amount of sodium carbonate x 100
