To prevent a condition called the “bends”, deep sea divers breathe a mixture containing, in mole percent, 10.0% o2, 10.0% n2 , and 80.0% he.
a. calculate the molar mass of this mixture.
b. what is the ratio of the density of this gas to that of pure oxygen?

1.       In order to prevent the condition called “bends”. He needs to calculate the Molar Mass of the mixture of 10.0% 02, 10.0 % N2 and 80 % He.
So the Answer is letter A. Calculate the molar mass of this mixture.
How to calculate:
Given Numbers : 10.0 % O2
                              10.0% N2
                               80 % He
Molar Mass = (.10 x 32g / mol.) + (.10 x 28g / mol.) + (.80 x 4g / mol.)
                          => 9.2g / mol.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

B.) falsehope this !