||Homework Help||Kindergarten-University. - Page 69

Originally posted by: Roses-khushi

can u tell me what is hydrogen bond????

Originally posted by: ..swagger..

wiki helps sometimes ACTUALLY

A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group."

Originally posted by: -.serendipity.-

Hi, is there anyone here who can help me understand percent abundance and relative atomic mass in chemistry? I've just stared and I'm kind of lost in this matter.

Originally posted by: akhl

I saw this question just now.

Percent abundance of an isotope of an element means what percentage of the element is present in Earth on the form of that abundance. For example nitrogen has two isotopes N-14 and N-15. The percent abundance of N-14 is 99.63 and the percent abundance of N-15 is 0.37. It means that if we take a sample of nitrogen, we will find 99.63% is N-14 and the remaining 0.37% is N-15.

Let us solve some problem on this.

Problem 1. Nitrogen has two isotopes N-14 and N-15. Exact weight of N-14 is 14.003074 g/mol and exact weight of N-15 is 15.000108 g/mol. Percent abundance of N-14 is 99.63 and percent abundance of N-15 is 0.37. Calculate the atomic weight of nitrogen.

Solution. Atomic weight of nitrogen = (14.003074)*(0.9963) + (15.000108)*(0.0037) = 14.007 g/mol.

So the method is to multiply exact weight and relative abundance of each isotope and then add these product values. Relative abundance means percent abundance divided by 100.

Problem 2. : Copper is made up of two isotopes, Cu-63 (62.93 u) and Cu-65 (64.93 u). Given copper's atomic weight of 63.55 u, what is the percent abundance of each isotope?

Solution. Let relative abundance of Cu-63 = x

Then relative abundance of Cu-65 = 1-x

(62.93)(x) + (64.93)(1-x) = 63.55

You will find x = 0.69

Or 1-x = 0.31

Therefore percent abundance of Cu-63 = 69% and percent abundance of Cu-65 = 31%

----------------------------------------------------

Relative atomic mass is defined as mass of the atoms of an element divided by mass of some standard.

When not specified, the mass of standard is 1/12 the mass of an atom of C-12.

So relative atomic mass equals the mass of the atoms of an element divided by 1/12 times the mass of an atom of C-12. The unit is unified atomic mass unit (u).