Which is the best example of the law of conservation of energy? O A basketball stops bouncing because its energy disappears.
O A rolling skateboard slows down because some of its kinetic energy changes to heat.
O A falling book hits the ground and all of its energy is destroyed.
O A toy car gains speed as it moves down a ramp because the ramp creates energy.

B

Explanation:

A rolling skateboard slows down because some of it's kinetic energy changes to heat

A. The complete balanced chemical reaction is:

PbS  +  1.5 O2  --->  PbO  +  SO2

B. First let us convert mass of PbS into number of moles. The molar mass of PbS is 239.3 g/mol, hence:

moles PbS = 29.9 g/ (239.3 g/mol) = 0.125 mol

From the reaction, we need 1.5 moles of O2 for every 1 mole of PbS, therefore:

moles O2 = 0.125 mol * 1.5 = 0.1875 mol

The molar mass of O2 is 32 g/mol, hence the mass is:

mass O2 = 0.1875 mol * 32 g/mol = 6 grams O2

C. Converting mass to number of moles:

moles PbS = 65.0 g/ (239.3 g/mol) = 0.2716 mol

From the reaction, we can produce 1 mole of SO2 for every 1 mole of PbS, therefore:

moles SO2 = 0.2716 mol

The molar mass of O2 is 64 g/mol, hence the mass is:

mass SO2 = 0.2716 mol * 64 g/mol = 17.38 grams SO2

D. First let us convert mass of PbO into number of moles. The molar mass of PbO is 223.2 g/mol, hence:

moles PbO = 128 g/ (223.2 g/mol) = 0.573 mol

From the reaction, we need 1 mole of PbS for every 1 mole of PbO, therefore:

moles PbS = 0.573 mol

The molar mass of PbS is 239.3 g/mol, hence the mass is:

mass PbS = 0.573 mol * 239.3 g/mol = 137.23 grams PbS