Question

If the reaction of 4.50 moles of sodium with excess hydrofluoric acid produced an 85.0% yield of hydrogen gas, what was the actual yield of hydrogen gas? Unbalanced equation: Na + HF “yields”/ NaF + H2 1.91 mol H2 2.25 mol H2 2.65 mol H2 3.82 mol H2

Answer 1

First  write a balanced  chemical  equation
that  is  2Na +2Hf  --->2Naf  +H2
This  implies  that  to  moles  2 of  Na  reacted  with 2 moles  of HF  to  form  2 moles  of  Naf  and  1  mole  of  H2
thus  the  mole  of  H2  =4.50  x1/2= 2.25moles  of  H2

Answer 2

1.91 mol H2 Let's rewrite that equation into a balanced equation. Let's start with the unbalanced version: Na + HF ==> NaF + H2 We have equal amounts of Na and F on both sides, but twice as much H on the right. So let's double the Na, HF, and NaF coefficients. 2Na + 2HF ==> 2NaF + H2 And we now have equal quantities of all elements on both sides, so our equation is balanced. Now looking at the balanced equation, 2 moles of sodium should produce 1 mole of hydrogen gas. So we should have 4.50/2 = 2.25 moles of hydrogen gas. But we only have an 85% yield, so we multiply the expected yield by the percentage, giving 2.25 mol * 0.85 = 1.9125 mol. Rounding to 3 significant figures gives us 1.91 mol which matches "1.91 mol H2"