Chemistry (Please explain)

Question

2Al(s)+3H2SO4(aq)→Al2(SO4)3(aq)+3H2(g)

1.) What minimum volume of 3.5M H2SO4 is required to produce 18L of H2 at STP?

 

Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)

2.) What minimum amount of 1.35M HCl is necessary to produce 27.5L of H2 at STP?

 

K2S(aq)+Co(NO3)2(aq)→ 2KNO3(aq)+CoS(s)

3.) What volume of 0.215M K2S solution is required to completely react with 185mL of 0.120M Co(NO3)2?

Answer 1

Okay, so start with these definitions -

1) the molarity of a base or acid is how many moles there are in one liter of the base or acid.

2) One mole of a substance is the number of molecules of that substance that weigh one atomic weight (the numbers on the periodic table)  

3) One mole of an ideal gas takes up 22.4 liters of space at STP.

 

PROBLEM ONE

 

So, starting problem 1 from the bottom -

A) We need 18L of H2 from the 3H2 that come out the right side of the reaction.

B) That means we need to end up with 18/22.4 moles of hydrogen.

C) We can see that there are 3 H2 molecules on the right and 3 H2SO4 molecules on the left, so, in terms of moles, we get 1 mole of H2 for every mole of H2SO4

D) So we need 18/22.4 moles of H2SO4

E) Every 1 liter of solution contains 3.5 moles, so we divide (18/22.4)/3.5 to get the number of liters of acid required. 

 

PROBLEM TWO

 

You do the same thinking for problem 2, but you have to notice that it takes 2 moles of HCL to create each mole of H2.

 

PROBLEM THREE

 

The third problem is slightly different. 

 

You notice that each molecule of K2S reacts with 1 molecule of Co(NO3), so you need one mole of K2S for each mole of Co(NO3).  Just calculate the number of moles in 185ml (remembering that's 185/1000 of a liter).  Then divide by the number of moles per liter of the other reactant, and you should get the number of liters required.