Help i need some last minute chem help I always stuff up my molar ratio - whether to times by 2 or divide by 2. does anyone have any tips that helps t (1 Viewer)

[21cchen]

i keep on stuffing up my mole calculations - whether to times by 2 or divide by 2. does anyone have any tips that helps to remember?

 

[XXXPUMPERXXX]

If you write the equation of the reaction before doing your calculations you wont miss it.

 

[jazz519]

Swap numbers in the equation when writing the molar ratio. Will never go wrong

For example,
C6H12O6(aq) --> 2CO2(g) + 2C2H5OH(aq)

Say we want to do molar ratio between carbon dioxide and glucose

Write it without the number first:
n(C6H12O6) = n(CO2)

Swap the numbers from the equation, I.e. 2 from co2 goes in front of opposite compound
2n(C6H12O6) = n(CO2)

Then just sub numbers and rearrange for whatever you want. It doesn't require any understanding doing this and always works


Another example:
3H2(g) + N2(g) --> 2NH3(g)

Let's compare H2 and NH3 do same as above Swap numbers I.e. 3 from h2 goes in front of NH3 and 2 from NH3 goes in front of H2

2n(H2) = 3n(NH3)
Rearrange for what you want

 

[21cchen]

Swap numbers in the equation when writing the molar ratio. Will never go wrong

For example,
C6H12O6(aq) --> 2CO2(g) + 2C2H5OH(aq)

Say we want to do molar ratio between carbon dioxide and glucose

Write it without the number first:
n(C6H12O6) = n(CO2)

Swap the numbers from the equation, I.e. 2 from co2 goes in front of opposite compound
2n(C6H12O6) = n(CO2)

Then just sub numbers and rearrange for whatever you want. It doesn't require any understanding doing this and always works


Another example:
3H2(g) + N2(g) --> 2NH3(g)

Let's compare H2 and NH3 do same as above Swap numbers I.e. 3 from h2 goes in front of NH3 and 2 from NH3 goes in front of H2

2n(H2) = 3n(NH3)
Rearrange for what you want

ok thanks ill try it

 

[CM_Tutor]

I think about it as a ratio.

Taking jazz's example, if I have fermentation, C₆H₁₂O₆ ---> 2 CH₃CH₂OH + 2 CO₂, and want carbon dioxide from glucose, it is:

​

where the ratio is carbon dioxide over glucose and so it equals the coefficient of carbon dioxide over the coefficient of glucose, and hence is 2 over 1.

Supposing that we had previously established that 0.123 mol of glucose reacted (say), we can continue:

​