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# Calcium hydride reacts with water to form calcium hydroxide and hydrogen gas. How many grams of calcium hydride are needed to form 8.5 grams of hydrogen?

Approx. ##170## ##g## of calcium hydride are required

##CaH_2(s) + 2H_2O(l) rarr Ca(OH)_2(aq) + H_2(g)uarr##

The balanced equation gives a 1:1 stoichimetry between calcium hydride and dihydrogen gas, We need to (i) work out the molar quantity of dihydrogen, and (ii) convert this into an equivalent mas of ##CaH_2##.

##"Moles of "H_2(g)## ##=## ##(8.5*g)/(2.0*g*mol^-1)## ##~~## ##4## ##mol##.

So, if 4 mol of calcium hydride are required. This gives a mass of ##4*molxx42.09*g*mol^-1" calcium hydride"## ##=## ##??g##