Is this true or not

Question

1u=1.660*10^-27kg

 relative mass of the Li=7u=1 atom=1.660*10^-27*7kg

7g=6.022*10²³ atoms of Li

1kg=1000g

1.660*10^-27*7kg=1.660*10^-27*7*1000g

1g=(1.660*10^-27*7)/(1.660*10^-27*7000)

7g={(1.660*10^-27*7)/(1.660*10^-27*7000)*7} kg

1.660*10^-27*7kg=1 atoms 

1kg=1/1.660*10^-27*7 atoms 

{(1.660*10^-27*7)/(1.660*10^-27*7000)*7} kg=(1/1.660*10^-27*7)*{(1.660*10^-27*7)/(1.660*10^-27*7000)*7}atoms= 6.022*10²³

Is this true or not

Answer 1

Prasenjeet,

 

Can't stress this enough: something that will help you keep your calculations straighter and easier to check is to include all units. If you keep track of your units, they'll keep you on track. So, you don't ever have just "1 mol", you have "1 mol Li atoms", etc. What this lets you do, is to see immediately if you made an error, because your result units will be absurd. If your result units are correct, then most likely you set up the math correctly (exception would be using certain numbers that are dimensionless = without units, but these are more common in engineering fields!).

By the time you get to the bottom you have kg = atoms. That's simply not *ever* true, without further unit descriptors; they are different sorts of data (mass vs. number count).

The reason this works is that these sorts of calculations are all by ratios and conversion factors. The latter *always* have factors -- write them out, don't be lazy! The numbers change if the units change! You never want to end up with mixed units (L psi kPa^-1, for example) -- a sure sign of error!

Answer 2

What, exactly, are you asking? Is what true? This is very convoluted.

There are errors in the calculations above:

line 2, the mass of an atom of Li is 6.94 u, but close enough

line 7, remove the "kg" from the end.

line 9, put parentheses around the denominator, otherwise you'll get an error

In the last statement, it looks like what you're saying is:

7 g = 6.022 x 10^23 atoms, which is true. 7 g is the molar mass of Li, which means that 7 g Li = 1 mol Li = 6.022 x 10^23 atoms Li.