Mole calculations
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How do you find the relative formular mass (RFM or Mr) of a compound
The sum of all the RAMS of the constituent atoms
What is the RAM
the relative atomic mass(the bigger number)
What is the percentage by mass of oxygen in MgO
16/40 = 40%
In the equation what does ‘n’ represent
• the number of moles • unit: mols or moles
In the equation what does RFM represent
the molar mass
In the equation what does ‘m’ represent
• Mass • Unit: grams
What is the equation for mass
m = n x RFM mass = number of moles x RFM
Give the different rearrangements of m=nxRFM
• m = n x RFM • RFM = m/n • n = m/RFM
What is the name for the number of atoms in a mole
Avagradro’s number
What is Avagradro’s number
6 x 10 23
What is the empirical formular
Shows the simplest whole number ratio in which the atoms of each element combine in a compound
A compound is found to contain 9g of Al and 35.5g of Chlorine. Determine its Empirical Formula
• Element Al Cl • Mass 9 35.5 • RAM 27 35.5 • Moles 9/27=1/3 35.5/35. = 1 • Ratio 0.333 1 • / by smallest 1 3 So the formular is AlCl3
How do you find the molecular formular from the empirical formular and the RFM
Add together the RAM’s in the empirical formular Divide the RFM by the total
What are the inaccuracies in the experiment to determine the formular of MgO
• Loss of MgO as white smoke from crucible • Incomplete reaction – hard to ensure all Mg had fully been oxidised
What are the improvements in the experiment to determine the formular of MgO
• Repeat to obtain consistent results and take an average excluding anomalies • Heat to constant mass
In the experiment to find the formula of copper oxide why do you pass a gentle stream of gas through the tube without lighting it
To flush out the air from the test tube.
What is the equation for percentage yield
Percentage yield = actual yield / theoretical yield x 100
What are the reasons for % yield being less than 100%
• reactions may not go to completion or could be reversible, • side reactions may occur, • loss of products in multi-step reactions,
What is concentration
Concentration = amount of a substance dissolved in a unit volume
What is the unit for volume
dm3
How do you convert from cm3 to dm3
divide by 1000
What are the different units for concentration
• g/dm3 (gdm-3) • mol/dm3 (moldm-3)
What is the equation for moles in solutions
number of moles = concentration x volume n = c x V
What are the different rearrangements for the n = c x V equation
• n = c x V • c = n/V • V = n/c