Posted by **sandy** on Friday, February 6, 2009 at 7:34pm.

Magnesium crstallizes in the hcp arrangment. The dimensions of the unit cell are height 520om, length on an edge, 320. Calculate the density of Mg (s) and compare it with the measured value of 1.738 g/cm^3

this is what i did :

m= 2* 24.30g/6.022*10^23 = 8.07*10^-23

v= l^2*h

v= (320)^2*520= .0053248

d=m/v

d= 8.07*10^-23/. 0053248

d= 1.51* 10^-20 cm

BUT the answer is suppose to be 1.75

I think I made a mistake on the volume :S

- density of Magnesium! -
**DrBob222**, Friday, February 6, 2009 at 7:52pm
You don't list units for all of the measurements but I assume the 320 and 520 are picometers. You must convert the volume you have, which I assume is in cubic picometers, OR convert to cm before you begin. I highly recommend that you convert 320 to cm and convert 520 to cm.

320 pm x (1 m/10^12 pm) x (100 cm/1 m) = 3.2 x 10^-8 cm BEFORE doing the calculations. See if that helps.

- density of Magnesium! -
**sandy**, Friday, February 6, 2009 at 8:10pm
sorry yes they were in pm I corrected my units:

m= 2* 24.30g/6.022*10^23 = 8.07*10^-23

v= l^2*h

v=(3.2*10^-8cm)^2* 5.2*10^-8cm v=5.32*10^-23cm

d=m/v

d= 8.07*10^-23/5.32*10^-23

d= 1.52cm

> it's still 1.52 where as the correct answer is suppose to be 1.75g/cm^3. Is my method of calculation correct?

- density of Magnesium! -
**drwls**, Friday, February 6, 2009 at 9:00pm
Is your volume formula consistent with the hexagonal close packed crystal structure? That might be the cause of your disagreement.

See http://www.chm.davidson.edu/ChemistryApplets/Crystals/UnitCells/hcp.html

- density of Magnesium! -
**Anonymous**, Friday, February 6, 2009 at 10:27pm
that just pretty much explains how to calculate height. and the length = 2r

- density of Magnesium! -
**drwls**, Saturday, February 7, 2009 at 10:30am
It says a lot more than that. The base area of a unit cell is not the square of 2r. It is a hexagon

## Answer this Question

## Related Questions

- chemistry - Gallium crystallizes in a primitive cubic unit cell. The length of ...
- Chemistry - An element has hcp packing with a hexagonal unit cell. the volume of...
- chemistry - An element has hcp packing with a hexagonal unit cell. Its density ...
- Chemistry - An element has hcp packing with a hexagonal unit cell. Its density ...
- Chem - Based on the number of atoms per unit cell and the mass of the atom, the ...
- General Chemistry - PLEASE ANSWER VERY SOON I AM STUCK! Crystalline silicon has ...
- college chem - metallic iron crystallizes in a cubic lattice. The unit cell edge...
- chemistry - This problem is from my chemistry book and I am having a little ...
- AP Chemistry - Titanium metal has a body-centered cubic unit cell. The density ...
- CHemistry....Please help - How would i calculate the density of Li2S in grams ...