chemistry questions

Question

A particular ore of lead (galena) is 10% lead sulfide (PbS) and 90% impurities by mass. What mass of lead is contained in 50.8 grams of this ore? Answer in units of g.

 

i thought the answer was 5.08, but i got it wrong, so how do i do this?? 

 

also, 

 

Assuming all volume measurements are made at the same temperature and pressure, what volume of hydrogen gas is needed to react completely with 3.52 L of oxygen gas to pro- duce water vapor? Answer in units of L.

Answer 1

Lead Sulfide is 207.2 AMU Pb + 32.065 AMU S = 239.265 AMU. You first need to determine what percent of PbS is Pb:

 

100 × (207.2 AMU PB)/(239.265 AMU PbS) = 86.6%

 

Now consider that 10% of the ore is PbS, which itself is 86.6% Pb. So you have

 

50.8g ore × 0.1 × 0.86 = 4.37 g Pb, final answer.

 

 

The other question is a stoichiometry problem. First, the overall synthesis reaction is

 

2H₂ + O₂ → 2H₂O

 

According to the balanced equation coefficients, 2 moles of H₂ reacts with 1 mole O₂ to produce 2 moles of H₂O, assuming 100% efficiency.

 

The gases at constant temp and pressure will react according to a 2:1 H₂:O₂ ratio, regardless of whether you are talking about moles or liters because all ideal gases occupy the same volume at constant temp and pressure. We can treat the gases in this problem as ideal gases. Therefore, you may set up the ratios and use the cross-product property of ratios to solve for the unknown volume of H₂.

 

2 L H₂/1 L O₂ = x L H₂/3.52 L O₂; x = (3.52 L O₂  2 L H₂)/1 L O₂ = 7.04 L H₂, final answer.