Michelle H.

Graduate Student at CU Boulder

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Chemistry

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Question:

For the reaction: Na2S2O3 + AgBr NaBr + Na3[Ag(S2O3)2] How many moles of Na2S2O3 are needed to react completely with 42.7 g of AgBr?

Michelle H.

Answer:

To answer this question dimensional analysis simplifies the question. In order to solve this the gram of AgBr must be converted to moles, then the molar ration of AgBr to Na2S2O3 must be accounted for to convert to the moles of Na2S2O3, and then finally the moles of Na2S2O3 can be converted back to grams. A simple way of thinking about this problem is g1-m1---->m2-g2 (this means that the grams of the first thing is converted to moles of the first thing, which is then converted to moles of the second thing, and finally converted to grams of the second thing. The molar mass for the for AgBr is 187.77 and the molar mass for Na2S2O3 is 158.11. To solve: 42.7 g AgBr x (1 mole AgBr/ 187.77 g/mol) x (1 mol Na2S2O3/ 1 mol AgBr) x (158.11 g Na2S2O3/ 1 mol Na2S2O3). It is important to write the units to see how the numbers cancel each other out through the dimensional analysis. We can see that the grams cancel out to convert to the moles and that AgBr is canceled out to convert to the moles of Na2S2O3. The moles of Na2S2O3 are canceled using the molar mass in order to solve for the resulting grams of Na2S2O3. The answer is 35.95 grams or 36.0 when accounting for significant figures.

Basic Chemistry

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Question:

Write the balanced chemical equation for the following reaction: Calcium carbide (CaC2) reacts with water to form calcium hydroxide (Ca(OH)2) and acetylene gas (C2H2).

Michelle H.

Answer:

To solve this problem first write a general equation with all of the species. CaC2 + H2O(water) ---->Ca(OH)2 + C2H2. Once you have the equations written you can then begin to look at the individual species. Balancing oxygen last. We have the elements Ca, C, H, O. Think about this an a math equation, the values on each side must be equal. Left----Right 1-Ca-1 2-C-2 2-H-4 1-O-2 This is the initial amounts, so face Ca and C are balanced so we can balance the H by added a coefficient of 2 in front of the water or H2, this will also balance the O. The answer is: CaC2 + 2H2O(water) ---->Ca(OH)2 + C2H2 We can check this by counting both sides again. Left----Right 1-Ca-1 2-C-2 4-H-4 2-O-2 Now we have confirmed that the two sides are balanced.

Algebra

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Question:

For what value of R does the equation 2x +Rx= 3 have 1 solution?

Michelle H.

Answer:

In order to solve this question the "x" can be factored from the left side of the equation giving the equation x(2+R)=3. Both sides can then be divided by "2+R" to isolate x thus resulting in the equation x=3/(2+R). Therefore there is one solution for all real numbers of R except (-2). The reason R cannot be equal to "-2" is because that would result in a division by 0 in which the equation would be undefined.

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