# Fuel for the Moon How moles, molar mass and mole ratios link grams to particles in a hydrogen–oxygen rocket engine, including limiting reactants and percent yield\. By Numa Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/YNca8FjcqVyl) ## Transcript ### 1. Fuel for the Moon This engine burns liquid hydrogen with liquid oxygen, the fuel of rockets built to carry crews to the Moon\. The equation on the stand says two hydrogen molecules react with one oxygen molecule to make two water molecules\. Engines are filled by the tonne, so engineers need a way to count molecules by weighing them\. Burning hydrogen makes only water\. The white frost on the pipes is ice from the desert air\. ### 2. A Mole Is a Count Chemists count particles in moles\. A mole is a counting unit, like a dozen, but one mole is 6\.022 times ten to the twenty\-third particles: Avogadro's number\. Each sample here is one mole of water, carbon or copper, and the balloon takes one mole of hydrogen\. Watch the balloon fill: the gas takes up far more room than the solids\. At one atmosphere, a mole of any gas fills about 24\.5 litres at room temperature and 22\.4 litres at zero degrees Celsius\. The line\-up is drawn three times life size\. ### 3. Weigh to Count Drag a sample onto the scale to weigh it\. The mass of one mole of a substance is its molar mass, in grams per mole\. Water is two hydrogens at 1\.008 plus one oxygen at 16\.00: 18\.02 grams per mole\. Mass divided by molar mass gives moles\. Moles times Avogadro's number gives particles\. The two\-litre bottle holds about 111 moles of water\. The small samples are drawn three times life size\. ### 4. Mole Ratios The numbers in front of each formula are coefficients\. They give the mole ratio: two moles of hydrogen react with one mole of oxygen\. But an oxygen molecule weighs sixteen times as much as a hydrogen molecule, so the ratio by mass is very different\. Below each formula is its mass: 32 grams of oxygen for 4 grams of hydrogen, about eight to one\. Hydrogen is so light that its tank is the giant one, yet the small oxygen sphere holds most of the mass\. ### 5. Static Fire An engine is tested on a stand before it flies\. Press Ignite\. Sparklers burn off stray hydrogen, then the valves open and the exhaust hits the flame deflector\. The bright diamonds in the plume are shock waves\. The white cloud is steam: water from the reaction plus boiled cooling water\. Sound is slower than light, so someone 300 metres away hears the thunder almost a second after the flash\. ### 6. The Limiting Reactant Drag the red valve lever to set the mixture ratio, O/F: kilograms of oxygen for each kilogram of hydrogen\. At the exact ratio, both run out together\. Below it, oxygen is the limiting reactant, the one used up first, and leftover hydrogen is the excess reactant\. Above it, hydrogen runs out first\. Too much oxygen would burn the engine's own metal\. The orange glow you see then is the hot nozzle\. ### 7. Why Run Fuel\-Rich? Press Compare to switch from six to one to the exact ratio and back\. Real hydrogen engines run at about six to one, below the exact ratio, so 24 percent of the hydrogen leaves unburned\. That makes the exhaust lighter, as the lower gauge shows, and lighter molecules leave the nozzle faster\. A little more hydrogen would push harder still, but the tank would grow enormous\. Six to one is the compromise\. ### 8. Percent Yield Press Run trap, then drag the cold trap onto the scale\. It was fed 32\.0 grams of oxygen and extra hydrogen, so oxygen limits\. The theoretical yield is the most product it can make: 36\.0 grams of water, at the red line\. Percent yield is actual yield divided by theoretical yield, times 100\. Yields fall short of 100 percent because some vapour escapes and some gas never reacts\.