Chemistry Tutorial 3.01b: Weight Average Atomic Mass

This video explains how to calculate the weight-average atomic mass of an element given the masses and relative abundances of the naturally-occurring isotopes.

Category:

Education

Tags:

• chemistry
• rosengarten
• weight
• average
• atomic
• mass
• nucleus

[tehrunescapefreak1]

I think you did your math wrong.... When you mutiply 35.0 x A PERCENTAGE, you have to move the decimal to make it a decimal and not a decimal. So it's 35 x .7553 divided by 100... no? which equals to 2643.55 not 26.

[MarkRosengarten]

@tehrunescapefreak1 Ask yourself...does it make sense for the average of two such small numbers to be such a huge number? If the average is not in the middle of the highest and lowest value, you have done it incorrectly. The decimal is the percentage divided by 100. The math is correct. Either you do 35 X 0.7553 OR it is 35 X 75.53/100...which is EXACTLY the same thing. If you are putting it into your calculator correctly. 75.53/100 = 0.7553. 35 X 0.7553 = 26.44.

[MarkRosengarten]

@tehrunescapefreak1 What you are doing is taking 75.53 % of 35, which is going to be less than 35, wouldn't you say? 100% of 35 is 35. 50% of 35 is 17.5. Do you understand?

[lukedaduke94]

thanks so much you have helpped me no end after having my first A-level chemistry lesson last week and i didn't have to ask the teacher and look stupid

[MarkRosengarten]

@lukedaduke94 Never fear asking a question. We can't learn otherwise. We have created a culture that makes failure not an option, though how can we learn and grow except through failure? I think we do ourselves a disservice. It is better to have tried and failed than never having taken a chance in the first place. Good luck to you and I hope you find other videos here equally helpful.

[Mexecution19]

u rock mr mark roengarten thanks for explaning this very helpful ,

[drpthemc]

@tehrunescapefreak1 Also, like MarkRosengarten said, would it make sense to have an average MUCH HIGHER than in between 35.0 and 37.0? Nope.

If you solved this problem your way, you would get an amu of 3548.94!!! WAY TOO HIGH.

Good luck.

[drpthemc]

@tehrunescapefreak1 i think you're confusing yourself when you start to move decimal places like your doing in this type of problem.

it's easiest to multiply 35.0 x 75.53 then divide by 100, which equals 26.4355 ...

then 37.0 x 24.47 then divide by 100, which equals 9.0539 ...

add the two together (26.4355 + 9.0539) = 35.4894 = 35.5 amu (adjusting for Significant Figures)

MarkRosengarten is correct.

[Iwantkababnow]

thank you so much!!!!!!!!!!

[tehrunescapefreak1]

@MarkRosengarten thanks! :) this was a very quick reply and a very helpful video! Just looking back to what i did wrong, mathematically thinking about it, is really stupid :( I wish I had a teacher as good as you haha. My honours chemistry class goes over 20 concepts in 1 day, and the teacher calls you out/yells at you for not paying attention if you ask questions lol.

[mares613]

thank you!

[amaliasayssmile]

Thanks so much!