The experimental apparatus represented above is used to demonstrate the rates at which gases diffuse. when the cotton balls are placed in the ends of a tube at the same time, the gases diffuse from each end and meet somewhere in between, where they react to form a white solid. which of the following combinations will produce a solid closest to the center of the tube?
(a) hcl and ch, nh
(b) hci and nhs
(c) hbr and chnh,
(d) hbr and nh

The combination of gases that will produce a solid closest to the center of the tube would be HCl and CHNH. Thus, A would be the right option.

According to the diffusion law, the rate of diffusion of gases is inversely proportional to the square root of their molar masses.

Following this law, a combination of gases that will diffuse from the opposing ends of the tube to form a white solid around the middle of the tube would be those whose molar masses are close to one another.

Thus, one can see that the combination of gases with the closest molar mass is HCl and CHNH.

More on diffusion rate of gases can be found here:  

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