these two words can both mean “where” 你在哪 = 你在哪儿= Where are you？（多一个“儿”是北方地区的习惯用语，儿化音） In addition， “哪”has another meaning “which” the main form is...

@Eddy167 No，it's not the same. 什么=what 哪 =which or where

@Eddy167 In fact， it's amost the same . “哪 nǎ” When you would like to pronounce “哪儿” more standard， you only need to lengthen some of the tail.

“nar” is ok.

@Eddy167 哪儿 is more common in northern and it was seldom spoken in southern.

@Eddy167 哪 na 哪儿 nar

哪儿，这个"儿"是中文特有的儿化音！