Aburning candle is covered by a jar as shown in the picture. the whole arrangement has a mass of 500g. what will be the approximate mass of the arrangement when the candle is completely burnt, after four minutes?

There's no picture, and nothing shown in it.  But from your description, and from performing the same experiment several times on my own over the years, I think I can answer the question if I try hard enough.

The "whole arrangement" means the jar and everything inside it ... the candle, some air, and something around the bottom rim of the jar, like a little bit of rubber or water, just to make sure that nothing can get in or out of the jar ... not even air.

You light the candle, and then you put the jar down over it.  You quickly and carefully measure the mass of the whole arrangement ... 500 grams.  Then you watch it, and you wait to see what happens.

After just a little while, the candle-flame goes out.  I mean, you're watching it, it burns for a while, then it flickers and it just goes out !  There's probably still some candle left to burn, but the burning just stopped !

What happened is:  If something is burning, then wherever you see flame, some of the material in the thing is hot, combining with OXYGEN in the air, new substances are forming, and then it needs more Oxygen.   Fire needs Oxygen to burn.  If there's no Oxygen, then it can't burn.  

With the jar over the burning candle, and no air getting in, the candle used up all the Oxygen in the air inside the jar, and when there was no more Oxygen, the fire went out.

What you need to know to answer this question:  The mass doesn't change !

At the top of the candle, there are chemical reactions going on. Some of the wax from the candle, and some of the linen or paper from the wick, are combining with some Oxygen in the air and making new substances that weren't there before ... like smoke, ashes, Carbon Dioxide gas, and Carbon Monoxide gas.

But everything that was inside the jar at the beginning is STILL inside the jar when the candle stops burning.  Nothing new came into the jar, and nothing went out of it. So the mass is STILL the same 500 grams.

what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.

bacteria that live on the ocean floor are sustained by phytoplankton. phytoplankton, microscopic marine photosynthetic organisms, have a vastly significant role to play not only in the marine food web of which they’re part of, but also on a more global scale. despite their infinitely small size in comparison to other marine organisms, these tiny creatures occupy an immensely important ecological niche: they are the foundation of the marine food web, and as primary producers, play key roles in supporting all other organisms in the marine environment, as well as in the regulation of the earth’s climate through the sequestration of carbon, oxygen production, and other related processes. phytoplankton account for roughly half of all global primary productivity; therefore, their significance extends far beyond the marine environment alone. there is an intriguing sense of irony in the realization that these tiny living beings that often live out their existence unnoticed and undetected by the rest of the world, have such a far-reaching impact on the lives of virtually all other living organisms on the planet, particularly on those in the marine environment. the world that we have become accustomed to has been and is continuously shaped by the workings of these miniscule yet vital “plants of the sea”.

hope this !