TBR test 2 physical sciences q2

[2010premed]

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What is NOT true about H2(g) and D2(g) at the same temperature?

• D2(g) has greater momentum than H2(g)
• D2(g) and H2(g) have the same kinetic energy
• H2(g) has greater velocity than D2(g)
• D2(g) and H2(g) molecules exert the same force when they collide with the inner walls of the effusion tube

A: D

So I get that at the same temperature, the gases must have equal KE, and that H2 is lighter so it has greater velocity than D2. But how can you pick between A and D? I don't get the way they do it in the explanation, is there a simpler/faster way? This is how they do it:


Because answer choices A and D both deal with momentum, the mv2 relationship in the expression of the average kinetic energy must be divided by v to obtain the momentum, mv. This is shown for each gas in Equations (2) and (3) below:

(2) mH2vH2 =

(3) mD2vD2 =

​

Given that the mass of hydrogen is less than the mass of deuterium, and that their kinetic energies are equal, the velocity of hydrogen must be greater than that of deuterium. This means that the denominator is larger in Equation (2) above (which represents the momentum of hydrogen gas) than in Equation (3). As a result, the inequality shown in Equation (4) below holds true (which means that the momentum of deuterium is greater and that choice A is a valid statement):

(4)

>

Therefore: mD2vD2 > mH2vH2
​

Because of this relationship, we know that deuterium gas molecules exert more force when they collide against the walls of the effusion tube than hydrogen gas molecules do. This makes choice D an invalid statement. The reason that H2(g) exerts the same pressure against the walls as D2(g), despite the fact that it exerts less force per collision, is that H2(g) moves with greater velocity, and thus collides with the walls more frequently

 

[Pons Asinorum]

Why didn't you just include this in the thread that chocolatbear started below about this same question? Why do you start so many new threads? Ahhh!

What is NOT true about H2(g) and D2(g) at the same temperature?

• D2(g) has greater momentum than H2(g)
• D2(g) and H2(g) have the same kinetic energy
• H2(g) has greater velocity than D2(g)
• D2(g) and H2(g) molecules exert the same force when they collide with the inner walls of the effusion tube

A: D

So I get that at the same temperature, the gases must have equal KE, and that H2 is lighter so it has greater velocity than D2. But how can you pick between A and D? I don’t get the way they do it in the explanation, is there a simpler/faster way? This is how they do it: ecause answer choices A and D both deal with momentum, the mv2 relationship in the expression of the average kinetic energy must be divided by v to obtain the momentum, mv. This is shown for each gas in Equations (2) and (3) below:

(2) mH2vH2 =

​

(3) mD2vD2 =

​

Given that the mass of hydrogen is less than the mass of deuterium, and that their kinetic energies are equal, the velocity of hydrogen must be greater than that of deuterium. This means that the denominator is larger in Equation (2) above (which represents the momentum of hydrogen gas) than in Equation (3). As a result, the inequality shown in Equation (4) below holds true (which means that the momentum of deuterium is greater and that choice A is a valid statement):

(4)

>

​

Therefore: mD2vD2 > mH2vH2​

Because of this relationship, we know that deuterium gas molecules exert more force when they collide against the walls of the effusion tube than hydrogen gas molecules do. This makes choice D an invalid statement. The reason that H2(g) exerts the same pressure against the walls as D2(g), despite the fact that it exerts less force per collision, is that H2(g) moves with greater velocity, and thus collides with the walls more frequently.