Weird but easy question.

[gochi]

This is a question from the OAT, but I heard the OAT/DAT are similar, since the sample exams have the exact same reading questions and some mc questions aswell.

If an electron has a mass of 9.709 x 10-31 kg,
and a proton has a mass of 1.672 x 10-27 kg,
approximately how many electrons are
required to have the same mass as one
proton?

A. 150,000
B. 1,800
C. 5.4 x 104
D. 5.4 x 10-4
E. 15 x 10-58


This is what I did...

9.709 x 10-31 : 1.672 x 10-27/1.672 x 10-27 = 9.709 x 10-31/1.672 x 10-27 =1

9.709 x 10-31 - 1.672 x 10-27 = 8.037 x 10-4 ... what am I doing wrong ?

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[Doogie Howser]

mass proton/mass electron. all you need to do is subtract exponents on the 10^x's and divide 9.7/1.67...

but realistically, you should immediately see that it is going to be some "number x 10^4" and since there is only one answer choice with that, move on

 

[gochi]

i did, a (10 x 10-31 / 20 x 10-27)= (10/20) x (-31-(-27)= 1/2 x -4...

 

[dentstd]

[nm. erased]

 

[dentstd]

But do 1.6E-27 / 9.7 E-31 and you get 1722. Not too too far from 2000.

Jumping the gun to an answer with 10E4 gives you the wrong answer here.

But remember which exponent is bigger, 10^-31 or 10^-27. Doogie made that mistake.

 

[gochi]

If an electron has a mass of 9.709 x 10-31 kg,
and a proton has a mass of 1.672 x 10-27 kg,
approximately how many electrons are
required to have the same mass as one
proton?

but we go (mass of electron/mass of proton)... with reference to the exponents -31-(-27)= -4...what the heck ? whats up with this ?

 

[dentstd]

no, you missed what I said. look at the numbers I used, and the order I used them. which one's on top?

an electron's smaller. if you divide Me/Mp, then you get a fraction. 1/1722.

if you divide Mp/Me, you get 1722. (The accepted true number is 1836)

 

[Doogie Howser]

oh haha whoops....i divided (subtracted) the exponents correctly but not the numbers themselves... silly me, its been a long day.

i hope i don't make that mistake on thursday

 

[gochi]

Yea, but it does not make sense that way.

If an electron has a mass of 9.709 x 10-31 kg,
and a proton has a mass of 1.672 x 10-27 kg,
approximately how many electrons are
required to have the same mass as one
proton?


"ONE PROTON" means --> (Mp/Mp) : (Me/Mp) --? 1 proton : (Me/Mp)

but what your saying through your calculations is that...

(Me/Me) : (Mp/Me) --> 1 electron : (Mp/Me)

 

[dentstd]

Yea, but it does not make sense that way.

If an electron has a mass of 9.709 x 10-31 kg,
and a proton has a mass of 1.672 x 10-27 kg,
approximately how many electrons are
required to have the same mass as one
proton?


"ONE PROTON" means --> (Mp/Mp) : (Me/Mp) --? 1 proton : (Me/Mp)

but what your saying through your calculations is that...

(Me/Me) : (Mp/Me) --> 1 electron : (Mp/Me)

You've overthought this til you've confused yourself. "One proton" means "1 Mp." "Mp/Mp" means "one."

If you want to think of this algebraically, Me*n=Mp. What is n?

I don't even know why you did the Mp/Mp, Me/Me shindig. Whatever role that played in your reasoning, discard it. I don't even understand your notation....combo of fractions and ratios?