Calcium hydride, cah2, reacts with water to form hydrogen gas:
cah2(s)+2h2o(l)→ca(oh)2(aq)+2h2(g)
this reaction is sometimes used to inflate life rafts, weather balloons, and the like, where a simple, compact means of generating h2 is desired.

how many grams of cah2 are needed to generate 145 l of h2 gas if the pressure of h2 is 823 torr at 21 ∘c?

Explanation:

To solve this problem, we can use the Ideal Gas Law:

pV = nRT

1. Collect the data in one place:

M_r:   42.09

           CaH₂ +2H₂O ⟶ Ca(OH)₂ +2H₂

p = 823 torr; V = 145 L; T = 21 °C

2. Convert temperature to kelvins

T = (21 +273.15) K = 294.15 K

3. Convert torrs to atmospheres.

4 Calculate the moles of H₂.

5. Calculate the moles of CaH₂

6. Calculate the mass of CaH₂

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.