Esther Lee 4H
Posts: 67
Joined: Fri Sep 28, 2018 12:29 am

so I worked out #2 on the practice midterm and got C22H44O22 but the answer is C24H42O21. Does anyone know where I messed up?

I first divided all percentages by the mass of the individual element. the smallest value after I did that was oxygen which was 3.15 so I divided all values by that number and got 1 for C, 2 for H, and 1 for O. With this formula, the molar mass is only about 30 when the question asked for 667 so I divided 667/30 and got 22.2 so I multiplied to get C22H44O22. I messed up somewhere but don't know where can anyone help?

Raphael_SanAndres3C
Posts: 33
Joined: Fri Sep 28, 2018 12:24 am

Re: garlic bread midterm practice #2

Most likely the error is from a rounding thing from somewhere in your calculations considering the number you are multiplying by is big yet the amount of error is pretty minor.

davidbakalov_lec2_2L
Posts: 61
Joined: Fri Sep 28, 2018 12:23 am

Re: garlic bread midterm practice #2

When you divided all values by 3.15, you should have gotten about 1.14 for C. This is not close enough to 1 to round, so you should've multiplied all numbers by 7.

Edward Xie 2E
Posts: 63
Joined: Fri Sep 28, 2018 12:23 am

Re: garlic bread midterm practice #2

You probably had a rounding error. The method I used was multiplying the molar mass by each percentage to figure out how many grams of C/H/O were present in glycogen, then I divided those masses by their respective element's molar mass to find the molecular formula.

Esther Lee 4H
Posts: 67
Joined: Fri Sep 28, 2018 12:29 am

Re: garlic bread midterm practice #2

davidbakalov_lec3_3A wrote:When you divided all values by 3.15, you should have gotten about 1.14 for C. This is not close enough to 1 to round, so you should've multiplied all numbers by 7.

oohhh you're right. duh. thank you!!

JiangJC Dis2K
Posts: 72
Joined: Fri Oct 12, 2018 12:16 am

Re: garlic bread midterm practice #2

You cannot round down. If you're going from that approach of multiplying the mass ratio of say carbon in co2 and dividing moles, then you'll need to multiply (basically guess and check) what integer will produce a next to close number. 0.15 is a little too high to round down since we are producing whole numbers.