HardChemistryHigh School (11–12)Atoms and Molecules › Laws of Chemical CombinationAtoms and Molecules › The Mole ConceptNumerical
At 110 °C and a fixed pressure, Meera sparks 50 mL of hydrogen with 20 mL of oxygen.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
At and a fixed pressure, Meera sparks mL of hydrogen with mL of oxygen. They form steam: . Which gas is left over, and how much? What volume of steam forms, and what is the total volume of gas at the end?
Need a reminder? The key ideaShow
Conservation of mass: in a reaction the total mass of the products equals the total mass of the reactants. Definite proportions: a compound always has the same elements in the same proportion by mass. Multiple proportions: when two elements form several compounds, the masses of one that combine with a fixed mass of the other are in a small whole-number ratio. Gay Lussac: reacting gases have volumes in a small whole-number ratio, at the same temperature and pressure.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Which runs out. mL of oxygen needs mL of hydrogen. There is mL, so the oxygen runs out first.
Step 2. Left over. mL of hydrogen is left.
Step 3. Steam. volumes of steam form for each volume of oxygen: mL.
Step 4. Total. mL at the end, down from mL at the start. Volume, unlike mass, is not conserved.
Answer: mL of hydrogen is left, mL of steam forms, and mL of gas remains in all.
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