How to find molecular mass?

How to find # of atoms, given molecular mass and percentage of one element?

  • The hormone testosterone has a molecular mass of 288.4 amu and contains 79.12% carbon by mass. How many carbon atoms are in each testosterone molecule? (A) 21 (B) 23 (C) 28 (D) 17 (E) 19 I feel like you have to know the empirical formula of testosterone to do this, but I have no idea how to get there.

  • Answer:

    you do not need to know the empirical formula for this problem. you first find rthe percent mass of carbon 288.4 x .7912 = 228.182 g of carbon in testosterone you then divide by the molar mass of carbon (12) to get the amount of atoms in 1 molecule 228.182/12= 19.015 = 19 atoms of carbon (answer (E))

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You just need percentages, not the formula. 79.12/100 = X/288.4 Cross multiply that bad boy and you get 100 x = 22818.208 x = 228.18 (or close enough) That's the weight of your carbon. Divide that by 12 (close enough) for the atomic mass of carbon and you get 18.998... Or 19 atoms of carbon per molecule.

Debra

you do not need to know the empirical formula for this problem. you first find rthe percent mass of carbon 288.4 x .7912 = 228.182 g of carbon in testosterone you then divide by the molar mass of carbon (12) to get the amount of atoms in 1 molecule 228.182/12= 19.015 = 19 atoms of carbon (answer (E))

Jeff

You just need percentages, not the formula. 79.12/100 = X/288.4 Cross multiply that bad boy and you get 100 x = 22818.208 x = 228.18 (or close enough) That's the weight of your carbon. Divide that by 12 (close enough) for the atomic mass of carbon and you get 18.998... Or 19 atoms of carbon per molecule.

Debra

You don't. You know that the total AMU is 288.4 and 79.12% of that is Carbon. So you have 228.2 amu of carbon per testosterone. The amu for carbon is 12, as seen on your periodic table. Divide 228.2 by 12 and you get 19!

This Site Might Help You. RE: how to find # of atoms, given molecular mass and percentage of one element? The hormone testosterone has a molecular mass of 288.4 amu and contains 79.12% carbon by mass. How many carbon atoms are in each testosterone molecule? (A) 21 (B) 23 (C) 28 (D) 17 (E) 19 I feel like you have to know the empirical formula of testosterone to do this, but I have no...

Reuven

You don't. You know that the total AMU is 288.4 and 79.12% of that is Carbon. So you have 228.2 amu of carbon per testosterone. The amu for carbon is 12, as seen on your periodic table. Divide 228.2 by 12 and you get 19!

0.7808 x 28 (for N2) + 0.2095 x 32 (for O2) + 0.0097 x 40 (for Ar) = mass of one mole of air

Lillie

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