# Homework Help: Calculating the mass and number of atoms of a structural I beam

1. Aug 22, 2008

### hsbc2003

Please add this title: Calculating the mass and number of atoms of a structural I beam.

1. The problem statement, all variables and given/known data
A structural I beam is made of iron> A view of its cross-section and its dimensions is shown in the figure, where d = 2 cm, w= 31 cm, h= 28 cm and the length (not shown) of the beam is L= 1.53 m.

The density of iron is 7560 kg/m^3. The atomic weight of iron is 55.85 g/mol and Avagadro number is Na= 6.02214e23/mol.

What is the mass of a section 1.53 m long? (in kg)
How many atoms are there in this section? (in units of atoms)

3. The attempt at a solution

I calculated the volume of the I beam ( (h*w*l)-(the volume of the empty space))
and then I multiply it by the density of iron to get the mass.

but the answer was wrong, thus i I tried to calculate the number of atoms using Avagadro's number the answer will be wrong as well....

I hope you help me calculating the mass...

Thank you
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: Aug 22, 2008
2. Aug 22, 2008

### tiny-tim

small earthquake in Chile … not many goldfish

Hi hsbc2003!

hmm … that's a really weird way to do it …

(and it's very easy to make a mistake that way … always choose a method that's resonably idiot-proof )

Hint: the whole cross-section has the same width (2cm),

so cross-section area A = 2cm x total "length",

and then volume = A x L.

3. Aug 22, 2008

### LowlyPion

Your method looks ok. What numbers did you get? What cross section? Did you keep your units straight?