SOLUTION: An iron atom has a mass of 1.79 × 10^−22 grams. A sample of iron has a mass of 9. 129 × 10^−3 grams? Determine how many atoms are in the sample using scientific notation.

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Question 1208781: An iron atom has a mass of 1.79 × 10^−22 grams. A sample of iron has a mass of 9. 129 × 10^−3 grams?
Determine how many atoms are in the sample using scientific notation.
a. 7.339 × 10^−25
b. 7.339 × 10^20
c. 5.1 × 10^19
d. 5.1 × 10^−24
Found 3 solutions by Edwin McCravy, math_tutor2020, ikleyn:
Answer by Edwin McCravy(20054)   (Show Source):
You can put this solution on YOUR website!

Divide and bring the negative exponent to the top and make it positive: Combine the exponents Edwin

Answer by math_tutor2020(3816)   (Show Source):
You can put this solution on YOUR website!

Answer: C) 5.1 x 10^19

Work Shown

n = number of atoms
(atom's weight)*(number of atoms) = total weight
(atom's weight)*(n) = total weight
n = (total weight)/(atom's weight)
n = 9.129 x 10^(-3) divide 1.79 x 10^(-22)
n = (9.129/1.79) x 10^(-3 - (-22) )
n = 5.1 x 10^(-3+22)
n = 5.1 x 10^19

If you don't have your calculator with you, or you're in a hurry, then you can take advantage of the fact there is multiple choice.
Focus on the 10^(-3) dividing over 10^(-22). As shown above, that evaluates to 10^19 which is present in choice C only. We can rule out the other options and don't need to calculate the 5.1

Another thing to notice:
9.129 is close to 10
1.79 is close to 2
10/2 = 5 which is close to 5.1

Answer by ikleyn(52776)   (Show Source):
You can put this solution on YOUR website!
.

For your info: Handbooks give the iron atomic mass 9.27*10^(-23) grams.

So, your professor (or your textbook) gives you wrong information.