<p>These are from the Official Study Guide for all SAT Subject tests, problems 4, 5, and 65 in chemistry.</p>
<p>Na2CrO4 - a soluble yellow solid
PbCrO4 - an insoluble yellow solid
NaNO3 - a soluble white solid
Pb(NO3)2 -a soluble white solid</p>
<p>A. Yellow solid and colorless solution
B. Yellow solid and yellow solution
C. White solid and colorless solution
D. No solid and yellow solution
E. No solid and colorless solution</p>
<li><p>Observed when 1.0 mol of Na2CrO4 and 2.0 mol of Pb(NO3)2 are mixed with 1L of water</p></li>
<li><p>Observed when 3.0 mol of Na2CrO4 and 1.0 mol of Pb(NO3)2 are mixed with 1L of water</p></li>
<li><p>The volume of the dry hydrogen gas at 1 atm and room temperature would be:
A. (25.2)(744.8+19.8)/760
B. (25.2)(760-19.8)/749.8
C. (25.2)(749.8-19.8)/760
D. (759.8-19.8)/(760)(25.2)
E. (760-19.8)/(749.8)(25.2)</p></li>
</ol>
<p>By the way, the answers are supposed to be A for 4. and B for 5 and C for 65, but I don’t understand why. Thanks for the help!</p>
<p>I'm not 100% sure of those first couple, but for #65:</p>
<p>Its going to be PV = PV
so volume of gas = 25.2, P of gas will be atmospheric pressure minus vapor pressure (749.8 - 19.8). And then the other side of the equation the pressure will be 760 since its 1 atm and then you solve for V</p>
<p>so its (25.2)(749.8-19.8) = (X V)(760)
X = (25.2)(749.8-19.8)/(760)</p>
<p>so I guess for #4 Na2CrO4 going to be soluble yellow solid in a lesser amount then Pb(NO3)2 which is soluble white solid. Pb and CrO4 will form a yellow solid and NaNO3 is aq. So Im guessing since you had a higher concentration of clear solution to begin with, you will get the yellow solid and a clear solution.</p>
<h1>5 you have a high concentration of soluble yellow solid and low concentration of clear soluble solution. so again PbCrO4 will make the yellow solid, but you have a higher initial concentration of yellow solution so its going to make a yellow solution with a yellow solid</h1>
<p>For both 4 and 5, you would get Na+, NO3-, Pb2+, and (CrO4)2- ions.</p>
<p>4) All of the CrO4 reacts with Pb to produce a yellow solid, PbCrO4, since you have only 1mol of CrO4, and 2 mol Pb2+. This is why you get A, the yellow solid, and colorless solution, because Na+ and NO3- are colourless, as is Pb2+.</p>
<p>5) The reason you would get a yellow solid and yellow solution is because not all of the (CrO4)2- reacts with Pb2+; some of it does, producing the yellow solid, and some of it remains with Na2CrO4, which is soluble and yellow in colour.</p>
<p>Thanks guys! I understand 4 and 5 now. I didn't realize that the two compounds would react with each other. For 65, how do you get 25.2 for gas volume and 19.8 for vapor pressure?</p>
<p>I'm still kind of confused. It doesn't given the numbers anywhere else so I'm guessing they're probably obtainable from solving from constants, but I have no idea what to do.</p>