What mass (in g) does 3.99 moles of kr have?

Answer is 334.35 g.

Explanation:

n = m/M

Where n is the number of moles of given substance (mol), m is the mass (g) and M is the molar mass of the substance (g/mol).

n = 3.99 mol
m = ? 
M = 83.798 g/mol

By applying the formula,
    3.99 mol = m / 83.798 g/mol
               m = 3.99 mol x 83.798 g/mol
               m = 334.35 g

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.