Question

What mass of potassium must react with water to produce 50.0 grams of h2?

Answer 1

The mass of potassium that must react with water to produce 50.0 grams of H2 is 1950 grams

calculation

Step 1 : write the equation for reaction

2 K + 2H2O → 2KOH + H2

step 2: find the moles of H2 that is formed

moles = mass/ molar mass

from periodic table the molar mass of H2 = 2 g/mol

= 50.0 g/2 = 25 moles

Step 3: use the mole ratio to calculate the moles of K

the mole ratio of K :H2 is 2:1

therefore the moles of K = 25 moles x2/1 = 50 moles

Step 4: find the mass of K

mass =moles x molar mass

from periodic table the molar mass of K = 39 g/mol

mass= 50 moles x 39 g/mol = 1950 grams

Answer 2

The reaction between water and a metal is vigorous and produces a metal hydroxide and hydrogen gas. First we write a balanced equation for the reaction.

`2K + 2H_2O \implies 2KOH + H_2`

First we find the number of moles of
`H_2` using the 50 g mass and the molar mass of dihydrogen 2.01grams.

`50 g H_2 * (1 mol H_2)/( 2.01 g) = 24.88\ mol H_2`

Then we use the moles of hydrogen and molar mass of K, 39.01 to find the mass of potassium needed to react.
`24.88 mol H_2 * (2 mol K)/(1mol H_2) * ( 39.01g)/( mol) = 1941.14 g K`

1941.14 g potassium should react with water.