Fill in the coefficients that will balance the following reaction:
+ → +

Our coefficients should be:

NaCl: 2
CaS: 1
Na2S: 1
CaCl2: 1

Why? 
 NaCl and CaS are our reactants. So, in other words, with these coefficients, we have 2 Na's, 2 Cl's, 1 Ca, and 1 S. To obey the law of conservation of mass, we need to make sure that we have an equal number of each element on the PRODUCTS side as well. So, if we have 2 Na's for example, we need to have 2 on the other side as well.

So, if we use the coefficient of 1 for Na2S, that tells us we have 1 S and 2 Na's. If we look back on the other side,we have 2 Na's and 1 S. So, this part is balanced and equal on both sides.

If we use the coefficient of 1 for CaCl2, we have 1 Ca and 2 Cl's.
If we look back on the other side, after the coefficients have been placed, we have 1 Ca and 2 Cl's on the reactants side as well.

So, now that each side of the equation has the same number of each element, it is balanced and equal.

Please let me know if you have any questions, I hope I could help!

Answer: abeexplanation:

1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.

Where is the graph ?