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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Joules23 on January 28, 2007, 10:18:53 PM

Title: Avogadro's Number
Post by: Joules23 on January 28, 2007, 10:18:53 PM

Solid tantalum is made up of repeating unit cells that are cubes, with edges of length 330.29 picometers. Each unit cell contains two tantalum atoms. Solid tantalum has a density of 16.654 g/cm^3. Calculate Avogadros number using this data. V=a^3

1 mole Ta = 180.9479g Ta = ? atoms Ta

i know im supposed to get 6.022e23 , but im not getting that, let me know what im doing wrong.

1 mole Ta=1 mole Ta (180.9479g/1 mole)(1cm^3/16.654g)((10e-2)^3/1cm^3)(1 molecule/3.3029e-24m^3)

= 3.2896e21

Title: Re: Avogadro's Number
Post by: enahs on January 29, 2007, 01:01:00 AM

Your units are wrong. And you are also not including the number of atoms in each unit cell.

N = (n*m)/(p*v)

N= avagardos number
n = number of atoms in smallest unit cell
m=molar mass
p=density
v=volume

Note, your molar mass and density are in grams and cm^3. This means you must either convert both of those into meters and kilograms, or convert the picometers into centimeters.

Title: Re: Avogadro's Number
Post by: pulse123 on January 29, 2007, 01:27:47 AM

Here is how I would work out the problem.

First, convert density to g/pm^3
(16.654g/cm^3)x(1cm^3/10^30pm^3) = 1.6654x10^-32 g/pm^3

Then, calculate weight of a unit cell by multiplying density by volume
(1.6654x10^-32 g/pm^3) x (330.29pm)^3 = 6 x 10^-22 g / unit cell

Since there are two atoms in the unit cell, weight of one atom is 3 x 10^-22 g

Finally, 180.949g / Avogadro's No. = weight of an atom = 3 x 10^-22g

Solve the equation and there you go. 

Title: Re: Avogadro's Number
Post by: Joules23 on January 30, 2007, 02:03:22 AM

thanks for that, it really helped me

i have one more question regarding Avogardros number:
Use the density of steric acid, the volme of a molecule and 1 mole=284g to calculate Avogadros number.

Density of steric acid = .1g/L
Volume of molecule = 8.89 x 10^-26 L

DxVxA=284g

(.1)(8.89x10^-26)A=284g
A=3.19x10^28

But according to my computer print out im supposed to get 3.4x10^24, what am i doing wrong?

Title: Re: Avogadro's Number
Post by: AWK on January 30, 2007, 02:22:38 AM

Quote

Density of steric acid = .1g/L

density is wrong

Title: Re: Avogadro's Number
Post by: Joules23 on January 30, 2007, 02:33:30 AM

wow yea.. your right, now i feel dumb, thanks