10% ethanol to gasoline?

[JasonE]

I can’t nail down how to calculate this question and would love to know so I’ll defer to those who do.
If I have a one gallon (128 oz) container and want to make one gallon of gasoline with 10% ethanol-----how many ounces of ethanol would I use? At first I thought it would be 12.8 ounces, but don’t think that’s right?????? Or would I start with 115.2 oz gas and then add 12.8 alcohol? Maybe I'm getting volume and weight mixed up?

 

[Dr.Peterson]

I can’t nail down how to calculate this question and would love to know so I’ll defer to those who do.
If I have a one gallon (128 oz) container and want to make one gallon of gasoline with 10% ethanol-----how many ounces of ethanol would I use? At first I thought it would be 12.8 ounces, but don’t think that’s right?????? Or would I start with 115.2 oz gas and then add 12.8 alcohol? Maybe I'm getting volume and weight mixed up?

Volume vs weight could be an issue, but let's ignore that.

Why do you think you're wrong? Aren't the two answers the same? And isn't it true that 12.8 oz is 10% of the 128 mixture you want?

You need 10% of 128 oz to be alcohol, and 90% of the 128 oz to be gasoline. And that's what you've got.

 

[Otis]

one gallon (128 oz) … Maybe I'm getting volume and weight mixed up

Hi Jason. You're measuring the liquids by volume (you're not weighing them), so the unit is 'fluid ounces'.

1 gal = 128 fluid oz

Your intuition is correct; gasoline that's "10% ethanol" means 10% of the blend is ethanol* and the other 90% is gasoline.

*Technically speaking, fuel-grade ethanol is a blend itself (that is, it's not 100% ethanol), but the difference is probably not an issue for you — unless you're selling blended gasoline.
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[Steven G]

What everyone said, including what you said, is correct.
The issue is that you are not sure if your work is correct. So let's divide the 128 fluid ounces in 100 equal part. Now each part has exactly 128/100 fluid ounces = 1.28 fluid ounces.
90% is pure gasoline and 10% is ethanol.
gasoline = 90% of 128 or 90parts*1.28 = 115.2 fluid ounces of gasoline.
ethanol = 10% of 128 or 10parts*1.28 = 12.8 fluid ounces of ethanol