Chemistry Exam Review

Question

Draw the Lewis structure for

A) BCL₃

B) N₂O₂

Answer 1

We can't easily draw on this platform, so I'll do my best to give a written description of how to draw the Lewis structures.

A) BCl₃: 24 valence electrons

Place B in the center with 3 Cl atoms singly bonded to the B. Place 3 lone pair sets of electrons (6 electrons) around each of the 3 Cl atoms. This uses all 24 valence electron. B does not have to have an octet.

B) N₂O₂: 22 valence electrons

Place the 2 N atoms in the center with an O on either end as follows:

O - N - N - O

Then place 3 lone pairs of electrons (6 electrons) around each O atoms and then place 1 lone pair of electrons (2 electrons) over each N atom. This uses all 22 valence electrons. But the 2 N atoms do NOT have an octet of electrons. To fix this, simply move 1 lone pair (2 electron) from each O atom to the adjacent N atom, and form a double bond between the N and O. It should look something like this...

O = N - N = O (with each O having 2 lone pairs of electrons)

Answer 2

What is the mass (in grams) of 8,102,000 atoms of Potassium (K)?

Solution:

convert atoms to moles.

1mole of K =6.022 * 10²³ atoms

==>8,102,000 K atoms * 1 mole/ 6.022 *10²³ K atoms==> 1.345 * 10^-17 moles of K

Step 2: Using molar mass of Potassium (K) from the periodic table--> 39.0983 amu= 39.0983 g/mol

we can now find mass of K

==> 39.0983g/1mol * 1.345 * 10^-17 moles==> 5.260 * 10^-16g of K

answer mass of K= 5.260 * 10^-16g of K