Stoichiometry uses coefficient ratios to relate moles of one molecule to moles of another

Calculating the mass of a product from the number of moles of reactants

APEX

calculating the mass of a product from the number of miles of reactants

Grams liquid × mol/g × Hfusion

Density (at a given temperature) is the parameter that relates the mass to the volume of any substance.

You need the balanced chemical reaction equation :
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2 K + 2 H2O ----. 2 KOH + H2

moles H2 = n H2 = ( 1.60 L ) ( 1 mol H2 / 22.4 L H2 ) = 0.07143 moles H2

moles K = n K = ( 0.07143 mol H2 ) ( 2 mol K / 1 mol H2 ) = 0.14286 moles K

mass K = m K = ( 0.14286 mol K ) ( 39.10 g K / mol K )

mass K = m K = 5.56 g of potassium <---

Answer : 5.56 g of potassium

The nearest gram is in the ones column, so we must look at the tenths column (the first number on the right of the decimal). If this number if 5 or higher then we must round up and report the value as 1.0 g or if it is lower than 5 then we report the answer as 0.0. All other numbers are irrelevant. In this case (0.[5]3525) the number we are concerned about (in the square brackets) is 5, therefore we round up and the answer is 1.0 g.