I need help with problems dealing with dimensional analysis.?

Question

here are the three i need help on:

1. the density of helium is 0.17 g/L at room temperature. What is the mass of helium in a 5.4 L helium balloon?

2. Liquid bromine has a density of 3.12 g/mL. What volume would 7.5 g of bromine occupy?

3. An irregularly shaped piece of metal has a mass of 147.8 g. It is placed in a graduated cylinder containing 30.0 mL of water. The water level rises to 48.5 mL. What is the density of the metal?

Answer 1

Okay so I kind of have to be studying but I love chemistry so here are the first two.

Density=mass/volume

1) 0.17 g/L = x g / 5.4 L... manipulate the equation D x V = M (0.17 X 5.4= 0.918 g)..... check your answer.. 0.918g/ 5.4 L = 0.17 g/L  YAY!

2) So heres a different method of doing it which I personally love using.. its called proportions..  If there is 3.12 g in 1 mL then there are 7.5 g in x mL

EX.  

3.12 g = 1 mL

         7.50 g = x mL          

                                        cross multiply 7.50 x 1 = 7.50 and divide

                                        by 3.12 and you get 2.4 mL..

Check your answer by seeing if each side or fraction (which ever way you look at it) equals the other ..ex.. 3.12/ 7.50 = .416 and 1/2.4 = .416  YAY..

two down one to go .. Hope this helps :D