# SCIENCE/CHEM

the purity of zinc is to be determined by measuring the amount of hydrogen formed when a weighed sample of zinc reacts with an excess of HCL acid.the sample weighs 0.198 grams. what amount of hydrogen gas at STP will be obtained if the zinc is 100% pure?

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1. Stoichiometry problems are about the same.

1. Write the equation and balance it.
2. Convert 0.198 g Zn to moles. moles = grams/molar mass.
3. using the coefficients in the balanced equation, convert moles Zn to moles H2.
4. Convert moles H2 gas to volume. moles x 22.4 L/mol at stp or if grams, then g = moles x molar mass. The problem just says what amount, not how many grams or what volume.

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2. Dr BOB,

Can you show me how to do the balanced equation please I am shocking at them
Thanks AndyWhats the simple way to explain balancing equations so i get it please....because i am horrible at them...
Thanks andy

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posted by Andy
3. Usually you have a word equation and you convert that to a symbol equation.
Zn + HCl ==> ZnCl2 + H2

Most of these can be balanced by trial and error. You learn to look for certain things. I would start this one by observing that I see 2Cl on the right and only 1 on the left. We can fix that with a coefficient of 2. (By the way, the first rule is that you may NOT balance the equation by changing subscripts. Once that equation is written as I have done above, the subscripts are set in stone.)
Back to the Cl. We can add a 2 to the HCl. (The ONLY way you CAN balance an equation is by changing coefficients. If one doesn't work we try another).
Zn + 2HCl ==> ZnCl2 + H2.
And it's balanced. I see 1 Zn on the left and right. 2H on left and right and 2 Cl on left and right. After you balance it you check it. You ALWAYS know if it is right or not. If it balanced it is right. If it doesn't balance it is wrong.).

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4. thanks.... andy

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posted by Andy

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